Go swimming at your local JCC during separate hours (yes, they exist outside of New York). Free for members & can be done anywhere.
Read library books. Free anywhere.
If you’re in Chicago, be sure to check out Ned Brown Forest Preserve, and in particular the Busse Forest Elk Pasture. It’s a great place to go biking, running, rollerblading, jogging, or walking. There are lots of elk and deer to observe. Bring a picnic and make a day of it.
There are two free entertainment complexes at Disney World (Orlando)– Downtown Disney and The Boardwalk. You can also ride the monorail for free. There are lots more suggestions in the Instructable “Best FREE Things to Do at Walt Disney World” (google to find it).