I found some old wheelbarrow coasters the other day in desperate need of a makeover, so I decided on piratey treasure maps!
Basically I printed the maps off on beige cardstock to make them look older, then sticky-backed them with double stick tape, modge podged the original coaster (for extra sticking power) and then modge podged the top (two coats) to seal it. After they dried I lightly sanded the edges with the toughest grain on my 7-sided nail file and was finished.
I'm not exactly sure how these will hold up over time, but we don't really use coasters often anyway so they'll suit my home just fine. I can tell you that the small stream of water I tested one under rolled right off.