Half Dome is about 18 miles roundtrip. If you miss the last shuttle bus back to your campground add another 2-3 miles depending on where your camping. It takes about 5 hours to get to the top. 4 hours to get back down. Take the Mist Trail to get to Nevada Falls! Prepare to get wet. There is water at the top. 2-3 active artesian springs and the Merced River. If you don't have a filter there is always someone who does and will share. I only haul enough water to get me to the top and retank before going down. Bring sunscreen, the top of the Dome is at 8800'. Only 2 bathroom stops along the way, so if your into a high fiber diet pack some TP and a means to pack out the used stuff. I make "poop kits." TP in a ziplock bag. I know this sounds gross but there are hundreds of people on the trail and if they all left TP in the woods, you get the picture. Bring a pair of gloves. You have to use steel cables to ascend the Dome and it is about an 800' climb! Climb on the outside of the path to get around the log jam. If I knew how to post a pic I would show you. Its intense! If you fall off its about a 4400' drop to the valley floor.
The Panoramic trail will get you to Half Dome. I did that one year. Glacier Point to Half Dome back down to the Valley. 23 miles! That is when I did my moonlight hike.
Bring lots of snacks.
Wow, I wish I was going too...