would love to find something close to a metro stop and walking distance to some sites. Please advise. I am not looking for anything luxurious ...just relatively cheap, safe and clean. We are traveling in August so A/c would be helpful. We are also only staying 4 days in each location so we could not stay anyplace that requires a weekly reservation.
Thank you in advance.

Head over to www.slowtrav.com they have a tonne of apartment reviews etc.

We used www.romepower.com to rent an apartment in rome in 2007 and they were very good.

We have used these folks in Venice.

www.veniceapartments.org

I think they may also have some offerings in Rome.

For Paris, there is a Paris Apartment thread here on Fodors with many recommendations and sites for you to check out. Just search for it.

http://www.vacationinparis.com/apts/id_182.htm

We stayed here in April 2008 and it's very close to the Odeon metro.

We found Tim Pearson and his agency, realrome.com very helpful. The apartment we rented in late Feb/early March did not have AC but you could check other listings.
I would not aim to be near the metro in Rome. It circles around the historic center and we used it only once in 8 days.