• Mount a rooftop antenna if you're into various forms of radio

CB, ham, shortwave, running a radio scanner, terrestrial broadcast digital TV, etc. Height is helpful there.

• You can probably get some kind of automated watering system, if that's your concern with the vegetables. Maybe need to fill a tank occasionally, but outside of that...shrugs.

• Some people like rooftop beekeeping. Don't need frequent access for that.

  

• Can throw some solar panels up there as a source of emergency power if the power goes out; if you put them above head height, then you've also got shade.

• I'm guessing that this is an urban environment ("access to" sounds like a multiunit building) so it probably has light pollution in the area, but if not, could maybe throw a telescope up there for unobstructed access to the sky. I've occasionally wondered about how hard it'd be to get some kinda motorized pan-tilt-zoom telescope-attached camera just to pan around and look at stuff on the ground on a monitor, and if you're looking at the city, it might be fun, kinda like adding an expansive view downstairs. I have some relatives that have housing with a nice view, and they liked to have tripod-mounted telescopes to look out and check out everything. In your case, probably want something remotely-operated, but same idea.