I'm another that doesn't recommend a guide. Nothing wrong with one but the Acropolis is easy to do on your own with a good guide book or info from the web.
I would recommend getting there when it opens, 8 a.m. to avoid crowds and lots of tour groups. At 8 a.m. it's quiet, peaceful and almost like being there 2,500 years ago (almost). Tours/tourists start around 9 a.m. and it just gets more crowded as the day goes on and becomes less enjoyable with pushing and shoving tourists wanting to get up close, the best photos and sometimes just obnoxious.
I would also recommend the Central Markets on Athinas St. just a short walk from Monistiraki Square. It's wild, crazy and quite an experience. Early morning is crowded with locals and taverna owners doing their shopping for the day but later quiets down but still stays busy.
The Meat/Seafood is on one side of the street under roof while the produce section is across the street in the open air.
Lots of street vendors also and many side streets with all kinds of small, locally-owned specialty shops in the area making it worth the time to visit the entire area.