You can absolutely visit a resort to eat at one of their restaurants. There is no charge for self-parking at the resorts, though there is a time limit of 3 hours. They may ask at the gate for your dining reservation #. (Not always enforced, but it's a possibility that they'll be enforcing when you're there.) Valet is $20ish, and I think doesn't have the time limit, though someone else who knows for sure might be able to confirm (or correct) that.
There is no charge for simply visiting the resorts, and many people enjoy resort-hopping while at Disney World - to eat, to see the decor, and to enjoy the theming.