I have road bikes. I am going to ride it arounf 122 miles, stay the night, maybe two and head back home. I want a decent backpack that will be comfortable with waist and chest straps and be able to hold a pair of shorts, tshirt, underwear and a couple of tires.
I looked on Amazon but there are dozens of packs so I figured I would ask here as I know someone here has done this before. I am not looking to pay a fortune either.

You might visit some area Thrift Shops.
I have found some good bargain bags in Goodwill.

Shimano Unzen 10

Even if you don't use the bladder, the Unzen series is worth it. I have an Unzen 6 that I use for similar purposes, and it is exceptionally comfortable. Unlike most other bags, the Shimano bags are designed specifically for cycling.

Thank You, was not aware Shimano made bags. I will take a look.

I wan't either until I found mine. Nashbar also has the Shimano Rokko, their "all around" backpack, which is a bit cheaper, and comes in simply horrific colors, like blue/orange and electric green.

Your not kidding about the colors, I'd be scared to hit a cyclist wearing a bag with colors like that.

Well, if you wear a red bandanna with the Unzen 10 in green you'll look like a Teenage Mutant Ninja Turtle.

But a tastefully restrained one.

I'm currently mostly using an older Camelback pack. It has a couple of pockets and straps, but still is reasonably compact. The bladder went bad a while ago but the pack is still handy.

Good suggestions here, but will they hold the items I mentioned in the OP?

I have two backpacks for commuting, one a lightweight casino give-away and the other a $150 tactical style for toting my laptop.

I paid $6 for each of them on eBay...

I forgot about flea bay.

Osprey backpacks, they make backpacks for cyclist as well.

Backpack, backpack....backpack, backpack.

Dora the Explorer

Go to the Closeout, bargain Bin, and Returned pages of Performance Bike and Nashbar. Yo will eventually find everything you want, andf half or two-thirds off.

Or, go to Ebay. Plenty of hydration packs from off-brands with plenty of carrying capacity.

For no more than you're saying you need to carry, you don't need a $39 backpack. You need one of these: https://tinyurl.com/gnth5oy

A sample is only 63 cents!

It would suck to wear a backpack for 122 miles. I'd get a cheap seatpost mounted rack for less than $30, put the stuff in a trash bag and tie it town.

Osprey Manta 36.
It's much larger than a lot of packs, but cinches down nicely and will easily hold a enough stuff for an over-night. It's a taller pack which means it comes down lower and curves along your pack so it's not bad while riding.

Honestly, go to REI and try a few packs. The problem with most biking packs is that they're oddly shaped, and don't hold very much. A 15L pack sounds like a lot except the bladder takes ~3L, and funny pack shape means you only have a ~8L left to work with. My ~12L Camelbak is basically full with only tools (just basic kit, medium seat bag worth), water, and a light jacket.

I had that thought as well...

I have the manta 36. Like all things osprey it is thoughtfully made and bomb proof. Use it mostly for hiking but also colder weather riding or when going to ride and picnic and need space.

I have the Osprey raptor 10 for cycling. Amazing bag, cool on the back and just enough room for the essentials plus some bits.

Osprey has the best warranty around also

Tie your stuff to the frame, get a handlebar bag, anything but a back pack.