There is a new contest underway on the Rimbit Forum at:  http://www.rimbit.com/threads/create-the-caption-contest.2363/

It's really straightforward, with the winner getting up to 400 RBT.

As long as you are registered on the Rimbit Forum you can enter (and registration is free).

If you're not registered, now would be a good time as these contests will likely continue.

There probably isn't an easier way to win free RBT.