My mom had one made just like what you are talking about, for my dad's shop...which is now my husband's shop and my pseudo-potting area. It was a little lame, but sweet. She did a picture of a dog, put his name on it and called it his "dog-house," for when he got fed up with her and had to get away to do "guy stuff." :)

I don't have any pix, but a mosaic plaque that they could all put some pieces on with you (and sign the back maybe) would be nice. Or, what about making stepping stones to the shed, out of those things you can buy at the home improvement centers (I think you use Quik Krete or something with them) and they could sign them or draw something in them while wet.

I just saw a post (I think on the cottage forum) about using an old mailbox for placing gloves and clippers to keep handy. Maybe this would be fun for your project. I would think it would be pretty simple to paint and let your friends "have at it" with paint pens or something similar. You could then place a clear protective coat over it for the long term. Just a thought. Let us know what you do and do share pics! Nikki