...to visit my grand kids. Any recommendations about rides there? I'm not a big MUP fan but I hear there are some really nice ones out there.

Road or mountain?
Climbing or flat?
It's difficult to recommend rides when we don't know what your preferences are.
They do have nice MUPs but can be quite crowded at times.

Road. Flat. Thanks!

If you want to ride the MUPs, all the bike shops in town have free maps. There are a lot of MUPs but they are more fun for transportation through town than actually going for a ride. They are also kind of convoluted, so be willing to be lost. Remember, the mountains are on the West. The Boulder Creek Path downtown gets very crowded on weekends.

Here are a couple of simple, flat-ish road rides. You can pretty much pick a road in the area between Boulder and Longmont, and it will be good. Neva-Niwot road will be under construction for a few weeks, it's still paved right now but they are going to resurface it. There are services in Hygiene, but not at Eldorado Springs.

https://goo.gl/maps/CEBK4
https://goo.gl/maps/xFGvs

https://goo.gl/maps/1jVVf
-- the southern 'spur' of this route from Baseline & Cherryvale to Eldorado Springs is the really nice part. You can return to town on the west side of town on bike paths, but it's too complicated to try to map. On the north loop, Valmont road is good too, but stay off Arapahoe.

Here is a moderate climb - you can start this one from the intersection of Hwy 36 and Lefthand Canyon if you want less riding. This ride is beautiful. There is a cafe at Jamestown, where you can get water and purchase baked goods.

https://goo.gl/maps/kuiil

You do realize that in CO when we say "flat", we mean "less than 6% grade", right?

At least the nice thin air will go in and out of my lungs easier.

Also, remember you're in high altitude. Take it easy the first couple of days.