None of the park is actually "closed" in May, though some of the roads are closed. For people with only a couple of days, that amounts to much the same thing.
Usually May is the best time for waterfall viewing, so at least one full day in Yosemite Valley will be superb. Depending on the stage of the Moon, an evening to see moonbows around the falls might be enough to encourage you to stay in (or at least very near) the Valley.
For a second full day, either the Valley again or the Mariposa/Wawona area is your best bet. As always, it depends on your likes. There's actually a trail all around the floor of the Valley which, at about 18 miles total, is great for two days for most people, one (long) day if you're really gung ho. There are also fabulous trails that lead up from the Valley as far and as high as you care to go. If you give us some idea of how you like to spend your time, I and many others on this forum can give you some more specific advice.