6.5 hour layover in Lisbon, Portugal
 

We are flying from Rome to U.S. and have a 6.5 hour layover in Lisbon during the day. Is there anything fun we could do/see during this time frame allowing for plenty of time to get back to the airport for our flight to U.S.?

There's a subway line that goes right into town, not sure how long the trip is, but you can probably get an estimate (it's 14 or 15 stops with a change of line, depending if you get off at Rossio or Baixa/Chiado). If you can get your bags checked through, and restrict your carry-ons, you'll be set--don't know if there's luggage storage in that airport. You should have time probably to get into the centre of the city, and just walk--perhaps take the Santa Justa Elevator up to Bairro Alto, enjoy the views, walk back down the back streets to Rossio. Make sure to find out when you have to be back at the airport to check in, and consider rush hour if that's the time you'll be there.)

Our first trip to Lisbon was a half day on a cruise ship and we had about 4 hours to walk around. It was very pleasant. Just spent a week there this spring, now we really love the city, but that few hours was a nice taste.

Personally, I'd buy a day pass at one of the airport lounges, such as TAP, and just relax for the time.

Trying to get to a city and back through security really isn't worth it considering the short period you have.

You have plenty of time to enjoy the city, the people, the cafes, the view, the river, and a must the castle of S. Jorge.
As the airport is close to downtown you may take the metro, the citybus or a taxi (will cost around 15 to 20 Euros).
Have fun

Thank all of you for your suggestions. We definitely want to get a taste of Lisbon rather than staying at the airport. We will look into going to the Castle of St. Jorge!! We are glad WWanderer mentioned restricting our carry-ons!!