I assume you mean a vascular plant as opposed to algae? Apart from mangroves and seagrass, there are no saltwater plants. A 5 min google search yielded nothing of use on brackish plants, so I wouldn't get my hopes up too much there. So as things stand you seem to have two choices: mangroves or a little nano/pico planted tank. You mentioned concerns about the size of mangroves... Well, bonsai would help you work around that for a while. Eventually it would get too big, but not for years. When it did, you could always transfer it to a 'fuge and get a new one. On the other hand, you could do a little pico/nano with macro algae and/or seagrass! Maybe one of the little Mr. Aqua tanks. You could pick just a few macro algae and let them get nice and big and then share the pics with us. :rollface: Maybe if you did one of the less common macros like Acetabularia in a smaller tank you could get it looking really nice and unique. Just a thought.
Of course if you are willing to go freshwater you could probably do a cool little mini-pond on your desk...
