A Cap N' Snap is a gacha-style gumball-looking machine where, using bottlecaps as currency, you can receive a random blueprint for a cosmetic or decoration. If you have a bottlecap in your inventory, simply interact with the machine to use it. There are only two machines per biome, but there are a lot of bottlecaps in the world, so collecting all the rewards isn't difficult. Related
Outbound: Painting Location Guide
Discover all 14 hidden paintings scattered across the four biomes in Outbound.