You will be able to find places to eat earlier than 10 (altho Nikki is right, that is when Greeks start to go out).

I found that the restaurants in Athens were open at all hours. I think they were mostly even open through the afternoon, unlike restaurants in Spain, for instance, where it was difficult to find anything open in the early evening. They just became crowded after 10 PM. Families with very young children were eating out in the restaurants we visited at 10:00 and later.

My husband and I spent 3 days/4 nights in Athens as part of a 3 week trip, 2 years ago. We were there in June of the Olympics year. I had read so much about how crowded Athens is that I was stunned when it really wasn't. Yes there were other people at the Acropolis. Yes there were other people in the museum. But there were many moments where we would be wandering around some of the areas and we would turn down a street only to realize that we were all alone. It was awesome. I wouldn't worry about crowds at all in May. (It was an interesting contrast when we visited Rome & Florence in September of last year - now those were crowds!)

There is plenty to see and do for 2 days. I truly enjoyed wandering around the central areas of Athens. They are very pedestrian friendly, there are lots of shops to look into (if that's your thing) and dozens of museums (if that's your thing). We still miss the cafes-you can sit outside sipping your drink for hours and watch the world go by (what else could you ask for on a vacation?!).

I, personally, would bring a bathing suit. Why not? It doesn't take up too much room. Your hotel may have a pool. You may decide that you want to brave the water. You may want to lay outside on a beach chair for awhile. At least you have it.

We stayed in Imerovigli and walked to Fira a few times. I wore athletic sandals during the day and comfortable but dressier sandals at night. I did lug sneakers all over Greece but the only time I wore them was to hike the Samaria Gorge in Crete.

Enjoy your trip, it sounds wonderful.