Almost everything you do with windows, but you don't have to worry about spending money on software, almost anything in the Linux world is free of charge.
~Coolest thing to do in Ubuntu 10.04? Don't install an anti-virus, you will never, ever need one again :)
~Watch a movie.
~Change the aspect of your desktop, make it look the way you want it too. You might want to install KDE and bathe in all it's eye candy, here is a somewhat recent review http://www.youtube.com/watch?v=Xr_MlWnGn4o
~You can make a happy dance because Linux uses fewer resources than windows, so your machine feels and works much faster, it's more responsive.
~Install boinc and do grid computing in your computers idle time. This way you help scientists solve some of sciences problems, and help humanity.
~Write a HUB about Ubuntu :P