The must have apps are ones only you can determine. Android tablets come with everything most people need, mail, browser, file manager, contact manager, music player and the list goes on. The only thing not in the stock OS is virus protection. I suggest AVG for light use.
Frequently, different sites will post lists of "essential apps". While subjective at times, you may want to Google that , to check them out. They generally give a description and a download link also. Last list i saw had roughly 40 apps that someone felt were "must have" , i think I had 5 or so.