if you don't normally ride in the rain, i'd wipe the bike down with warm soapy water and a sponge, paying attention to the underside and making sure no dirt was accumulating. i might hit it with some spray car wax after that (basically wash it like a car). then check the chain and all of the axles for signs of water intrusion

if it's just a beater, dry it off and make sure the chain is lubed