Ubuntu 9.04 (and 8.10, I think) has a "Guest Session" option. To get to it, you need to have the Fast User Switching applet on your panel, then click it, and click "Guest Session". The guest has no sudo and no privileges, so it is a safe place to let whoever do whatever. It also doesn't keep any files saved while in it, so there is no cleanup. If you need to get back to your session without ending the guest session, press Ctrl+Alt+F7 (defaultly) and then enter your password, and to go back to the guest session press Ctrl+Alt+F8.

I agree. 9.04 is almost a regression for many things, and would have been better as a "service pack" / bugfix release than trying to add so many broken features.