We went on the chuckwagon horseback ride and bbq in Yellowstone and took a float trip in the Tetons. A little pricey, but if you are looking for something special/different.
In all parks, lots of beautiful short hikes/walks for all ages. Be sure to take advantage of ranger talks/walks. You will get a newspaper/activity guide at each park listing all of the programs, but if you want to plan in advance, visit nps.gov; each park has a lot of information listed and you can usually download the current park newspaper/brochure.
