OK Guys. I've tried this and it actually works !!! If you have any paint lying around, sandwhich some paint between your foam
pieces. Works like a charm. I would stick with a latex based paint. The lacquer based paints will eat up the foam.
I'm going to be putting two sides of a tombstone together so I can't have anything that expands. I know it's a little late to be working on this, but hopefully I'll have a few done before the big night.
Call me crazy, but I've used hot glue on every single one of my foam pieces. Some of the softer fluffier foams will melt even on a low heat setting, but most have been fine with it. Even used the high heat setting on a couple with no ill effects. A large piece like two tombstone halfs would be hard to do fast enough for the glue to not harden, but for small areas and things like gluing on moss or decorations it works pretty good. Just use low heat, keep the tip away from the foam, and test an inconspicuous spot first to see it the foam will tolerate it.
I think I used 3M foam adhesive when I made my Pumpkin Hearse out of white foam. I built it several years ago and it's still solid. It did take a while to fully cure (several days to a few weeks??). But I did have some scraps that were glued together which I attempted to pull apart after it fully cured. I was unable to break the bond of the foam adhesive and the styrofoam gave way before the adhesive. I basically pulled the foam apart and the adhesive stayed intact.