My daughter was recently given an easel with a dry-erase board on one side. She scribbled all over it with crayon and when I went to clean it off, the marks proved to be quite stubborn. After trying spray cleaner, soap/water and considerable elbow grease with no success, I remembered the Magic Eraser - a microfine sponge that's a wonder on hard surfaces. It worked like, well, magic! It's also great for removing juice stains from plastic cups and poster paint "ghost clouds" from linoleum (not recommended for use on wooden play furniture though - it'll gently sand off the clear topcoat).
In the states I think there's a Mr. Clean branded magic eraser, and here in the UK I got mine from Woolworth's. I find that having one around pre-empts the worst of my clean-up stress, so my daughter and I can both enjoy her playtime. Hope that's of use to someone else!
I've used Magic Erasers on wall and floor scuffs with good success. Any other suggestions?