You don't have to spend a specific dollar amount. However, the certificates come in $10 amounts and change is not given. If you use a $10 certificate to buy a $7 plush, you will NOT get change. Therefore, you may want to add on something else to your purchase to bring the amount over $10 and pay the difference, just to get your money's worth out of the certificates.
If you want to buy a $50 watch and use $30 worth of certificates, you'll be able to do that and you'll pay the $20 difference.
I have $140 worth of certificates which will be used toward the renewal of my annual pass (yes, even though I don't get room discounts now). I'll just be paying the difference between $140 and whatever the AP renewal rate is.
On the website, there is a drop-down menu once you access your account called "account actions." Select "manage rewards" from this menu. On the page that opens, click the blue bar at the bottom that says "manage rewards." In the window that opens, select "order reward items" from the drop-down menu. That will give you a list of items you can order.