The GeekGold coupons you hand out at BGG.con and elsewhere ... a quick adjustment to the site code would allow you to embed the code in the URL for insta-redeem (e.g. http://boardgamegeek.com/geekgold/coupon/redeem?code=ABCEDFG...), and then you could put the QR code for that URL directly on the card so they could be scanned quickly.
I have 11 coupons from the con. Each with a 29-character hashcode to enter manually. Entering all that is a bit painful.
This is a great idea. Failing that, it would be nice to either eliminate similar characters (Zero and Oh, One and Ell) or just to break the codes up with spaces (consider transcribing "3UEH173BQ9WARQECP5R5YA8RRZR733" vs. "3UEH1 73BQ9 WARQE CP5R5 YA8RR ZR733").