I often think I'll just do some body weight moves or go ride my bike, but by the time my kids go to bed I'm beat. I finally gave up and have started doing push ups and lifting games with my kids (3.5 and 2.5 years old). Sometimes they ride my back while I do push ups, and, of course, they're always happy to be tossed up, leg lifted, or any number of things we can do with them.
There's a book that specifies, with pictures, many movements that can be done with small kids, or while pushing strollers:
Sarah Fragoso's Everyday Paleo. It has a lot of other things in it as well, including a lot of recipes.