It will be hard this late, start calling the tpwd number (if you want a state park) tomorrow 512/389-8900 for thursday I would not use the internet ressies personnaly
Both Ray Roberts parks are good and have a beach,there are reviews on all those parks on this forum. We like Cooper Lake SP (south), look for the park name or do a search on the left. Good luck
There is a discussion on internet ressies on this forum too
If they are full try ceder hill sp its close and a very big park here is a recent trip
Cedar Hill SP 4/08