Well, public services will be closed and maybe banks will have some days off, too. But public transport will be running pretty much to schedule and most all touristy things will be open, indeed restaurants, cafes and night-venues will be bursting with people!!
So by all means I would go. Personally, I would recommend SKG for a side trip (2-3 days) because it's quite an interesting city, in addition to being the country's undisputed culinary capital (plus, I am from there so a bit biased
).
There are plenty of things written on ATH if you want to do a forum search, and I'd happily answer questions on where to stay/what to see/do etc if you gave me an idea of your likes, budgets and so on...