Some Teton trails are pretty flat, though. A walk around Leigh Lake and/or String Lake would be relaxing and scenic. Or rent something to get out on the water.
If you have never been to Tetons, it is definitely worthwhile and not to be missed as long as you're so close (Yellowstone). If you've been there before but haven't been to Glacier, then go visit Glacier.
The way I think of those 2 places is, Tetons inspires me to sit and enjoy the view, and Glacier makes me feel like driving or hiking. But that's probably just me.