EVEn in winter, the Acropolis is open every day ... and so will the NAt. Arch. MUseum also open Sundays... altho it might close early. The ACropolis Museum (a pvt institution not part of Nat. Mus & MOnuent system) will also be open on Sunday.... so no reason for ou to hang out in dorm unless u want to. ??
BTW, you probably don't want to do a formal Tour, so you should know about the RIck Steve D-I-Y FREE Audio tours -- a step by step guide to Acropoilis , to the ancient Agora, (the ruins ribht below), aand the NAt. Arch. Museum (on this one he tells u the Top 10 HIghlights so u don't get lost in the vastness). Where do u find them?? Right from this RS PAge -- heres how;
• In upper LH corner click "Explore Europe" then CLick "GReece" and then "AThens"
• On the Athens page, clilck "LIsten" and y ou'll see the list of 4 free Audio step by step gides u download onto a smart phone -- since I'm guessing u haven't prepped much & don't have time, good to listen to them before u go to familiarize yourself.
What knd of day out? REally, Athens has SO Many sights/treasures, that a week wouldn't be enough to cover them all ... GO the the CYCLADIC Museum, really stunning... And Hydra is a summer thing ... a looong trip on a closed-in ferry, not fun.