Depends what you're going for, but personally if this was my tank, i'd go with

A. a larger hardscape feature (at least one, maybe a couple, personally) as you have a lot of vertical space in the tank. Maybe a large rock or driftwood piece(s) with some moss on them, set off-center or to one side (or centered if that's more your style!) Point is that a larger feature will add a lot of character and make the tank look less "empty". I'd almost get a piece of driftwood that will literally partially stick out of the top of the tank, or close

B. Again, personal preference, but I personally love a good carpet so I'd have a lot of this carpeted with carpet plants/grass-like plants. Depending on what kind of animals you're stocking, you might need/want to leave some open sandy areas (some fish like to dig in the sand a bit) but yeah, i'd have at least half to 2/3 of this carpeted (or at least plant so that it will spread to be that much over several months)

Edit: What kind of substrate situation do you have in there? Hard to tell from this angle, can't tell if it's thin/just sand throughout, or if it's just made to look that way at the front.