Both the Pohono Trail (from near Tunnel View) and the Panorama Trail (from Nevada Fall) go up to Glacier point, but both are pretty long for a go & return day hike. I think the Pohono is just under 14 miles one way, and the Panorama is about 8.5 miles to Glacier from Happy Isles. Both trails are outstanding for views and scenery. I don't know much about the opening and closing of those trails, I've usually done them in fall just for the reasons you mention...the summer season hits before the upper roads and trails open. Maybe someone else knows if the Panorama trail is accessible before Glacier Point road is open (?) It's not too steep, and I can't think of any areas that might present a severe ice problem, but that doesn't mean there aren't any.
Mariposa road (highway 140) is the most likely to be open of all the highways in, because of the lowest elevation. All 3 (120, Big Oak Flat, 140, and 41 (Wawona) are open all year, but 41 is the most likely to close for snow.
Thanks for the photo comments. Yes, last year we hit it nicely with our Half Dome trip at the end of May and having Glacier Point open to boot.
Even if Glacier Pt. road is closed, you won't be too depressed, I'd think. In late May the dogwoods are still blooming, the falls are huge, the meadows are turning green, rivers are roaring...there's not a lot to complain about 8^).
Gary
Yosemite Photo Galleries: http://www.pbase.com/roberthouse/yo