You absolutely must go to Rocky Mountain National Park. The scenery is absolutely eye-popping. Unfortunately Trail Ridge Road probably won't be open all the way through yet, but the trade-off is that the crowds won't be ridiculous. You can still get part-way up Trail Ridge Road and get a taste of the park. Bear Lake is accessible year-round as well.

I live near Nederland and agree that it's worth the drive from Boulder as well. Boulder Falls is about halfway up the canyon and is a nice stop. There isn't much in Nederland itself though, so you won't need more than an afternoon.

IMHO, there isn't much to do around Vail if you're not hiking or skiing, especially in early May. Ski season will be over but the summer activites don't get started until June. Worth the trip, sure, but I probably wouldn't spend more than one night there. Camp Hale is about a half-hour scenic drive from Vail, and the Colorado Ski Museum is in Vail.

BTW - Boulder traffic will be a mess during graduation. If you can, try to stay somewhere within walking distance.