Some cities have a household hazardous waste disposal location where one can drop off used chemicals, paint, etc. They will separate stuff that can be reused and properly dispose of the rest. The one in our city, we just backup our car and they take out the stuff while you sit in the car and complete the paperwork and show an id.

You can also dump in sand or kitty litter. You just need to make sure that when they run the compactor in the truck it doesn't explode paint everywhere.