Micronations

Micronations are self-proclaimed independent states that are not recognized by other countries. Often whimsical or artistic, they are usually small and eccentric enough to fly under the radar of the "real" countries they're located in, and many are tourist destinations.

Stay safe
Some micronations offer to stamp your passport on entry. It may be wise to use an expired passport for this, to avoid possible hassles from border control officers in the future.