If he's got stuff that looks like plants in his tank then those are corals. I wouldn't get him a fish. Some fish don't get along with others while some eat corals if he has any.
Gift cards are always nice to have. If you choose to do a gift card, then find out which fish store he likes to go to or even Petco is a good one. H is always in Petco getting supplies (Petsmart doesn't carry much saltwater stuff so I wouldn't get one for there). Plus Petco price matches anyone online. MIL got H a gift card to his favorite fish store one year and he was super excited.
A really nice tank scoop net? Get it on Amazon, and see what other items come up as "people who have purchased this have also purchased..." and maybe you'll get inspired.