Theoretically POTUS go to anyplace and see anything he wants to.  The question is "does he want to?".  Remember, plausible denial is everything to a politician; he can truthfully deny something he never saw, heard or did.  Area 51 would be easy, since it's a closed military reservation.  He could fly in with a minimum USSS protective detail, travel in one of the USAF's small biz-jets, meet and greet the Large Greys, and give them all a MAGA cap to wear in a day trip.  HAARP in Alaska, the same only wearing his muk-luks.