If you don't have a specific purpose behind your videos, and can't think of any ideas, give up now.
If you refuse to give up, try a video game, or multiple games. PC games can be recorded with a hypercam, camstudio is my favorite because it causes very little lag. A lot of Console games now have the option to save and record all your games, which allows you to post them on youtube as well. Video games are popular, and as long as you are good, or can make a good guide, as well as having some editing skill and the ability to add commentary, then people will want to watch them.