If it's very busy, they might not let you self-park for anything other than a meal reservation. If it's not too busy, they'll let you in for any decent reason (other than "I just wanted to take the monorail to the park"

), including "I just want to look around." Usually there will still be a 3-hour limit unless you valet park.
Actually, I've heard that at super-busy times they might not even let you self-park for a meal reservation!

In those cases, you have the choice of going back where you came from and taking Disney transport or a cab, finding the nearest open parking lot and taking DT or a cab from there, or valet parking.
Does anyone know if they ever close so tight they won't let you valet park?