Looking for ideas for a solo bike tour this summer in the northeastern US. I am based near Albany and have so far considered riding to NYC, Boston, Maine, Montreal, or Buffalo. I like the idea of combining multiple of these destinations into one ride, for example Albany to Montreal to Buffalo (through Canada) and then back to Albany. I think I will be able to manage 100-125 miles a day for up to 9 or 10 days. I don't much experience right now with multi-day tours but will do a couple smaller things before this. I have done a decent amount of distance in single day rides, though; my longest is 250 miles last year. Any advice on where to go, potential routes, sleeping on the go, etc.? I am under 18 so I cannot check into campsites or hotels.

Albany to NYC is feasible as it has some on road mileage from Albany to Kingston, then it’s possible to use rail trails all the way to the Bronx, for 100 miles or so. Not much camping on that route though, which as you stated is OK as you need to motel or warm shower stays.

Another suggestion is the Lake Champlain loop, north from Albany to Whitehall, then up the east side in VT. of Lake Champlain to Rouses Point, then down the west and NY.

Third suggestion is the St Lawrence River loop, up the NY or Ontario side and back the other. Or extend and up to Montreal, then back down. Very scenic and flat. Nice riding along there.

Issue I see is how to make hotel reservations. Maybe as the day progresses and you know you can make a destination, call a family member to make the reservation. Problem is, will they let you check in if under age.

I have not been under 18 for so long, I do not recall what the limitations are. But you have mentioned many of the options already. Last summer I planned a Buffalo to Kingston to Niagara Falls trip that was planned to go farther, but a heat wave similar to the one we have right now hit and I quit early. I wrote up some on that trip at this link:
Bike Tour near Lake Ontario and part of Erie Canal - June 2024

I do not know if someone under 18 is allowed to camp at the lock and dam stations along the Erie Canal. Some of the lock and dams allow camping for bicyclists. I camped at one such location on the trip I cited above.

I was forwarned for this year that a lot of the Canadians that take their RVs down into the USA for camping in the summer are staying in Canada this year. So this year might have crowded campsites in Canada. It has been my past experience that even in normal years, the Canadian Provincial Park campgrounds can fill on weekends after Canada Day (July 1).

Your plans for average distance per day are for more miles than most do for bike touring. I rarely exceed 75 miles in a day. If you are hauling a lot of camping gear on your bike, that does tend to slow you down. Not so much on flat ground like the Empire State Trail system, but in hillier areas it can slow you down a lot.

For those who are unaware, state facilities often (if not always) require at least one person in a party to be 18 or older. I have also seen this requirement at numerous private facilities.

Is under 18 still an issue for youth hostels? E.g. https://www.hiusa.org/find-hostels#eastern-region

I started my touring in college (but over age 18) and would make frequent tours from Boston into broader New England. There are even fewer hostels now, but at the time found them to be not too expensive place to stay. When not in a hostel, I sometimes stayed in motels but often brought a tube tent and camped beside the road. I was doing longer distances and found it easier to ride many of the daylight hours and camp not too long before dark. My longest gonzo trip was Boston -> Montreal -> Riviere du Loop -> Houlton -> Bangor -> Boston with 1194 miles in 9 days (8 days riding and 1 rest day to explore Quebec City). So between hostels and camping beside the road it was also less expensive

I have had three "life memberships" in my life perhaps reflecting different stages: (a) a life membership to Hosteling International (b) a life membership to Adventure Cycling and just recently (c) a life America the Beautiful Senior pass to National Parks/Monuments/etc.

When I worked at an HI several years ago, unaccompanied minors 16-18 could say at the hostel so long as they had a state issued photo ID, letter from parent/guardian, and contact info (generally phone number) of parent/guardian. But Hostelling International's US network of hostels has been decimated over the past five years due to pandemic, which was already a pale shadow of its pre-9/11 network. Non-affiliated hostels may have a similar policy, but it'd be on a hostel-by-hostel basis. In any case, it would be very difficult these days to do a bike tour in the US and string together enough hostels that would be within biking distance from each other. The only place that would fit the bill would be in Banff and Jasper National Parks in Alberta, but that's not in the US.

That would have been difficult even when I was doing it and even more now. I was thinking more of an occasional "anchor point" in cities like NYC, Boston or Quebec City where other parts of the trip could be camping.

The other exception when I started touring was Cape Cod. At the time there were half a dozen hostels including one on Nantucket, one on Martha's Vineyard and ~4 on the Cape itself. Average distances were perhaps ~30 miles apart. There were organizations that put together trips for teens in greater NY area (forget if it was Connecticut or Long Island) where they would get two leaders and a dozen or more teens and go for a week from hostel to hostel up and down the Cape. It brought a certain energy and filled up the hostels. Though even at age 19, it could be taxing to be in a hostel filled with 14-year olds and all their concerns... Looking at the hosteling page there only seem to be three hostels left on the Cape and the islands.

At the time, I also found hostels in Maine and New Hampshire but they were always less busy than ones on the cape.

If you're truly adventurous you can always head up into the Adirondacks and plan on camping primitive, such as Moose River Plains and many places like that. Check out Freecampsites.net. Or Stealth camping. Stealth camping opportunities are limitless up in those mountains.

Good advice. There's a ton of first come, first serve primitive sites in the Moose River area, as well as Horseshoe Lake, the Floodwood Road and the Jessup River Wild Forest. None of those require checking in of any kind, anybody of any age could grab a site.

Albany to Buffalo on the Erie Canal Towpath has free camping at the locks.
Back when I was <18, most of us had some sort of bogus ID so we could buy liquor. Not that I would suggest breaking the law. Given today's political climate, international travel with phony ID might be injudicious.
Maybe a ride around Champlain, or into Vermont?

Check out the Empire State Trail and the East Coast Greenway. The Empire State Trail basically begins in Manhattan, goes up north to Albany, and from there, one part of it swings west and goes all the way to buffalo, and the other continues North to the Canadian border. I haven't personally ridden it so I don't know the conditions. The East Coast Greenway is a mix of trails and roads that run all the way from Key West to Maine. It also passes through Manhattan before continuing on into New England, so you can probably plan a route where you ride one to NYC and get on the other. The ECGW can have some gnarly road sections, my understanding is that it's only REALLY bad down South. I've ridden it between Phily and NYC and felt it was fine, other than a few miles above Philly. The have a map on their website and the sketchy portions are highlighted red.