I've been going to Rehoboth since I was very young....now we've bought our own beach house there and go 1 to 2 times a month, in and off-season. Of course, I enjoy the town more in off-season (especially Sept-Nov). In the fall, the weather is great (usually) and there's lots of great activities. DO NOT miss the SeaWitch Festival before Halloween...or the Christmas concert and tree lighting Thanksgiving weekend - both right downtown.
As far as parking...yes, it can be tough in season. Many of the side streets are permit only. BUT, they added an electronic system (think iPhone app) so you no longer need quarters 9and all the running back and forth). Simply download the parking app (directions on the meters), load your car, credit card, etc. info and from then on you're good to go. Simply enter the "zone" number on the meter, choose how long and off you go. You can check how much time you have left at any time and add time through the app. It even texts you as you get close to expiring. Never have to run to feed the meter again!