Most of the standard shade trees should be fine, like maples, oaks, lindens, sycamores (London Plane), elms. If you want them close together, look for columnar varieties, which are easy to find in maples and oaks. Lindens are a little narrower than other shade trees, too.

Sycamores and some maples will grow faster to give you that thick trunk you want for hanging the hammock, but I think the sycamore will be too wide for what you want to do (unless the hammock is hung by long cords!).

Be careful with maples to pick ones that can handle our alkaline soils, or they'll likely get iron chlorosis, which makes the leaves turn yellow and retards the tree growth. Most of the plain green varieties are OK, except for silver maples. I have some that turn red in the fall (Autumn Blaze), and they look so cool in the fall that it's tempting to get them, but they have suffered some chlorosis. One of them was probably going to die until I put some iron implants in the trunk. You don't want to have to mess with that kind of care.

And it's probably obvious, but I would avoid any of the conifers, because they look tacky when people trim their lower branches up away from the ground.

Sycamores are a messy tree, Locust trees are the strongest trunk trees per inch out there. That is why they use them around State buildings. So they will handle a hammock at a younger age than most. Just be sure it is a non fruiting hybrid so you dont have the seed mess to deal with.