You cant really just turn it into an Android and using a browser for youtube would be difficult. The best thing for videos would be to use the LeapPad Manager
app to download youtube videos and convert them to be installed to the device. The app can handle all of that.
You can get simple flash games
installed. I only did one as a proof of concept. There are a few others posted in the Innotab forums that should work on the LP as well. I don't remember if I tested them on the Ultra though.
What games / apps do you have installed? I did manage to get Doom
and Duke Nukem
running on the Ultra, but that is not for the faint hearted. It sure took a TON of work setting up an environment to compile, finding source code, modifying it to work with the LP buttons etc. I believe I got Hexen and Heritic working as well and packaged them up into a single .tar installer.
Finally, if you know your way around Linux, you can get Shell access
to poke around and mess around on the device.
Also note, if you install custom apps / games, they will get wiped out if you connect to the PC or WIFI.