I still vote for Geosyncronous Transfer Orbit (Using the launch stage, not the CubeSat itself), with an Estes booster to raise the Perigee or Apogee when needed. But One thing's for sure: A Kerbal Space Program Community Publicity Stunt needs to have an Estes motor. A C6-P motor would provide plenty of impulse, and it's be dirt-cheap. An Electrostatic Propulsion system for a 3U cubesat would increase the cost immensely, not the least because the CubeSat itself would cost much more. If we can't go for GTO, a Moon flyby with an eventual gravity-assisted De-orbit would be cool. Especially with a camera to observe the Moon. I really don't think we should aim too high for a publicity stunt. But if you insist on breaking the boundaries, Then Phobos would be the ideal mission, but we would need to accommodate the extremely expensive Electrostat. Best case scenario we land on Phobos and we take pictures from the surface. Even better if we can manage to do some mass-spectrometer and take some sample of the surface. If only Earth had a Minmus orbiting it. Then we could throw a Cubesat at Miinmuus. If Asteroid Redirect Mission happens IRL before we complete even building the CubeSat, We could rendezvous with the Asteroid. L1 or L2 wouldn't be at all fitting for a Kerbal Community mission, for Kerbin has no Lagrangian points. And, as the Official Jebediah Kerman of the Community Space Program, I DEMAND Model Rocket Engines!