Some already give discounts to past passengers via coupons or reduced prices on balconies, but I hear ya! I'd much prefer a shipboard credit to a cocktail party (I don't drink and have yet to go to one of these) or bag/souviner (donate them to Salvation Army or give to kids on shore).
I like the coupon books that are placed in your cabin, but it seems they have become less and less "lucrative" as time goes on. Here are some that I like:
Free Ben & Jerry's
Behind the scenes tours
I think it would be nice if they had a coupon like :
X % off your sign and sail. It could be capped or tiered with maybe even something like first $10 free.
Free "upscale" dining (don't know if I'd use it, but I definitely won't pay for the alternative upscale dining)
...but I think my favorite one, which would be within reach of being offered, would be "free soda during your cruise!"
Hmm, I seems to have a theme of "free" in my append :-)