Okay, first, you don't get to pick admission vs. a Fun Card. If you do not already have valid admission of some form (AP, multi-day pass) on your actual birthday, you will be given free admission.
Second, IF you already will have valid admission (you mentioned being there for a few days so I am guessing you will), you can select an alternate birthday gift (free, admission ticket good for 1 day within the coming year, fun card, or fast passes).
The Fun Card does not expire and you cannot get change back.
Some key can and cannot use tips:
- You can use it at DL, DCA, or select shops iin DtD (like WoD). There are some shops in DL that are NOT owned by Disney. They do not accept the Fun Card. (Ex: The Crystal Shop)
- You can use it toward buying/upgrading an AP.
- You cannot use it towards a hotel bill.
- You cannot use it on food (as in dining), but
you CAN use it on food merchandise (like pre-packaged candy from the candy store).
Full details are
here, but I am guessing that's the legalese you were referring to.
- Dreams
P.S. Welcome to DIS!