My parents recently tried to buy me something dive-related for my birthday. They even went to my LDS and asked the shop for help. The guys at the shop tried to come up with something and ended up saying that they couldn't think of anything they had that I didn't already have. Honestly, I can't think of anything reasonable that I want but don't have. (If I were among rich people, a doubles rig would be a nice idea, but it's got too many zeroes for our social class.)
My parents bought something anyway. When they gave me the present, before I'd even opened it, they said I probably already had it, and that the shop said I don't even need a receipt to bring it back for credit. They just wanted to give me something tangible, even if only as a placeholder. Turns out, I already had their gift idea covered in spades, so I have a small store credit now.
For a fairly new diving couple getting married, there are all sorts of possibilities. If they're more seasoned, the only diving options are trips (in whole or part), consumables (fill cards and such), and diving-related gifts (art and so forth).
I'd love to do a "pay for our trip to Bonaire" thing when the opportunity eventually arises, but people tend to spend more on a gift than they'd give cashwise. Hmm... registering only at places that have liberal return-for-cash policies, then? :biggrin: Or even better, take the remaining unreturned gifts and have a special eBay sale, so people can contribute to the trip *and* get a lovely thank-you gift.
(Unless it's some really nice All-Clad stainless. I can never get enough of that -- I'm a kitchen nut, too.)