A fly in is just that, you fly in, have a good time, then leave, there isn't an aerial display or program of flying displays other than just a few blokes or ladies going for a flight or two. An Airshow is an event put on by an organisation that have everything from port-a-loos, first aid personnel and fast food stalls, they have an enormous insurance bourdon, the fly in is far more relaxed, how ever I'm sure if someone wants to take legal action against you for what ever reason, they will, advertise it as a fly in, come one come all, let the pilots know what is available, fuel, underwing camping, airfield situation, frequency if any, power, approaches, local attractions and aviation hazards. Put out a mud map so folks know what to expect, contact details and have a great time.
Most importantly, let me know when it's on, and I'll be there.
Good luck.