The Mist Trail doesn't offer anything approaching the heights of the Grand Canyon for sure, but there is one place where you're walking a skinny path with the mountain on one side and a fence on the other just before reaching the top of Vernal Falls that could definitely be uncomfortable for anyone nervous around heights. One very good thing about the Mist Trail is that it's easy to turn back if things are going wrong, since the return is all downhill.
I definitely second driving to Glacier Point. The waterfalls will be AMAZING that time of year, and you'll have great views of at least three of them (Yosemite Falls, Vernal Falls, and Nevada Falls). I can't remember if you can see Ribbon Falls from there, as it was dry by late August when I went last time. I think Illiluoette Falls is visible from there too, but the first three are the really spectacular ones.
After going to Inspiration Point and seeing it from a distance, you might want to see Bridalveil Falls up close. It's an easy and flat 1/4 mile or so walk from the parking lot, and is pretty cool to see when the flow is high as it will be then.
If Tioga Pass Road happens to be open by then, that's also a great drive you guys should consider (all the way out to Mono Lake and back). You get to drive alongside a pretty cool canyon at the end, and get great views of some of Yosemite's iconic peaks like Half Dome, Clouds' Rest, Cathedral Peak, Mount Hoffman, Mount Dana, and many others. Plus there's three really beautiful lakes, Tenaya Lake, Tioga Lake, and Ellery Lake, along the drive.
EDIT: It looks like Tioga Pass Road has been open by May 25th in 7 of the last 10 years, so hopefully luck will be with you if that drive interests you!
http://www.nps.gov/yose/planyourvisit/tiogaopen.htm