We had seasonals for a few years before we had to upgrade to annuals so we could go at the holidays.
Seasonals do not include parking.
Seasonals do include discounts at some restaurants and stores. Some of the restaurant offers are for lunch only, and the list does change. For stores, last time we had Seasonals it was World of Disney and Arribas Brothers only....both at Downtown Disney. That's what it is for Annuals now, but can't promise on Seasonals.
You can always upgrade later to an Annual if you decide to go during a blackout period. What they do is credit what's left on your Seasonal to the purchase of the annual. So instead of paying the difference between then and your original expiration date, you buy a new annual pass, good for 365 days, and the give you a pro-rated credit for what was left on the old one. Not sure exactly how they determine the credit amount, but it always seemed fair when we did it.
You get a newsletter also, that sometimes includes invitations to special events. For instance, we were able to explore Asia in Animal Kingdom one morning before it was officially opened. Access was by showing your pass only, and when we got in we had a light breakfast (free) since it was early (juice and donuts, I think it was). Pretty neat perk, I thought.
HTH,
Katie