If you don't get a VERY early start to Half Dome (6AM or earlier) you can expect it to be crowded from the granite steps of sub-dome to the top, and back down, which is not generally fun. The trail itself isn't bad though, as people are somewhat spread out over the miles.
A couple of alternatives; park at the base of the 4-mile trail, head up to Glacier point, then down the Panorama trail to the John Muir and mist trails back to Happy Isles, where you can take a shuttle to your car at the 4-mile trailhead. That's more than I really like to do in a day, but it's feasible if everyone's in good shape, and it's a fantastic hike. Probably about 13 miles, but the first 4.5 are very 'up'. The mist trail back will give your knees a workout, and you'll be ready to call it a day, I'd guess.
The other would be the Upper Yosemite Falls trail and on to Eagle Peak, then back. The Upper Yosemite Falls trail is a toughie, but the nice part is that if you get to the top and everyone still feels like they're in good shape 8^), you can continue on to Eagle Peak. If not, you can head back down, and I've never known anyone to do the upper falls trail and back and not be beat by the time they're back. Eagle Peak and back to the valley are about 12.5 miles, very strenuous. The upper falls trail itself is only 7-8 depending on where you go at the top, but it's not easy. Another fantastic hike though.
The Half Dome trip is great, but if you can't get to the trailhead early, may not be so wonderful, which is why I suggested the alternatives. For the Falls trail, you might have to park near the village at day use parking, and take the shuttle to Camp 4/sunnyside where the trail begins. Or possibly park at the Swinging bridge picnic area, cross over via the walkway to the Lodge and over to Camp 4.
Gary
Yosemite Photo Galleries: http://www.pbase.com/roberthouse/yo