I'd suggest a good guide book; the Colosseum is in Rome.
Seriously with just 2 full days in Athens, and 2 teenagers (which you haven't mentioned since your January inquiry), you need to strategize. You haven't said you are booking a tour of the Acropolis (not Colosseum) or the Ancient Agora, or the Acropolis Museum. Most tour groups hit all at once -- about 9:30 AM -- so in summer it's terribly crowded going up on "the Rock". Thus we always suggest beating the crowd by getting to entry at 8:15. However, you have teens, and lots of us remember how hopeless it was getting them up dressed & out early! The Answer? D-I-Y ... and on this very website (click Watch Read Listen & type Greece), you can download FREE FREE step by step audio guides to both Acropolis AND ancient Agora. (if you don't know what the latter is, click Wikipedia or that Guide book i suggested).
After Acropolis u can walk across street to Acropolis Museum.. I hear they have audio guides at the desk (if not already taken when u get there). It's air-conditioned, & you can have lunch at glassed-in terrace looking right up at Parthenon. For better understanding (if u do not have guidebook or audio) I suggest going straight to 3rd (top) floor and watching a 20-minute video about Acropolis/Parthenon history (partly documentary, partly animated VERY well-done). Then u can go down to 2nd floor exhibits (u may even be able to watch craftsman working on statue restorations.
What other things for day 2?? The Nat. Arch. Museum is HUUUGE - you can get lost in rooms of clay pots & never be heard from again -- but again, RIck Steves can help, with FREE audio guide to this Museum's Ten Top Dont-Miss sites.. you could do it in an hour. What else, for the kids? You need to say boys or girls, or both. The tech minded would love the Kotsanas Museum of ancient Greek Technology - http://kotsanas.com/gb/index.php o... (worlds first computer 500 BC!) The jewelry-minded will swoon over the Lalounis Jewelry Museum -- just yards away from the Acropolis in an elegant Mansion - http://www.lalaounis-jewelrymuseum.gr Lalounis was world-famous, like combo of Tiffany/Cartier... gorgeous gems, sometimes u can see craftsmen making gold items.
Of course all these things and many more are in a guidebook -- free even, at your local library.