Deep water soloing and water bouldering! Great for the Texas heat! Some very good walls can be even accessed without a boat, although having a boat makes the experience much more enjoyable. Make sure you check the landings, especially if the water level is low! Some of the rock can be chossy, test your holds before committing.
Grades and route length can vary due to water level, so the listed grade and height is more of an approximation then anything else.
If you don't have a real boat, the best system I've found is to get an inflatable raft, hike in with it in a backpack and then blow it up with a hand pump once you get near the walls. This saves you from having to do some very sketchy hiking (maybe 4th class for some people?) while carrying an unwieldy inflated boat. It is also alot easier to start most of the climbs from an inflatable boat instead of an inner tube or the like.
If you decide to use an inflatable raft (very convenient and recommended unless you have a real boat), make sure you take enough life preservers for everyone in the boat because it is common to have the lake patrol come check you while on the water.
For the areas that can be reached by hiking, park at the Soldiers Bluff park (free) off of Hwy 22, next to the dam.
Mountain Project's determination of some of the classic, most popular, highest rated routes for Lake Whitney (DWS):