Well, you kind of covered everything the sites offer... Questions, answers, flagging, editing, voting, closing, reopening, deleting... did I miss one? Chatting (just kidding, chat doesn't benefit anyone).
I suppose you could use the review queues, but I had assumed that was related with your mention of flagging content, closing content, or editing content.
The best way to improve the site would be to simply improve the way you use it. Do more research for posting questions or answers, make more edits which significantly improve posts, help close questions that need closure either in the queues or through flagging from the newest list, vote on posts where you feel you can determine if the content is of use or not; increasing any of these activities will benefit the site indirectly, and if everyone were simply doing this the sites would greatly benefit.