A mosaic is a form of 2-dementional art in which small pieces of glass, stone, tile or other materials are arranged into pictures or patterns.
To help spark some ideas, here are a few examples of LEGO mosaics, all found on LEGO Quest. (LEGO Quest has a list of 52 LEGO Challenges that would be a great reference for future LEGO club meetings of ours as well.)
Castle--This one is built vertically--if that makes sense-- so you can make one like this even if you don't have a large flat base for building on like the rest of the examples.
Snowy city with a church and house
Another option is to create a geometric pattern.
Or even just your name.
I guess LEGO actually sells a "Mosaic" set--though its completely not needed in order to do mosaics, but here are a few fun examples people posted on Amazon of what they made from their kits.
Have fun mosaic-making--we'll see you there.