I think I read somewhere that you can "pre-buy" your CD and get a discount. You don't have to actually buy the cd first, just register for it at the photopass website when you get home, pre-order the CD, and then go back online and put in your photopass numbers from the cards you rec'd in the park. Don't know if this is still true or not.
Also, I didn't realize while there, and its a good tip. You can have as many photopass cards as you want and put them all on the same acct for ordering. So, if you and DH get split up (like one is riding with an older sibling,etc.) don't hesitate to continue getting photopass pictures. Just get another card from the first photographer you see. When you get home, register all the cards and you will be able to get all of your families photos on a CD (or several depending on how many pics you have.)
Have a great time!