We are a group of 4 adults who have 12 hours to kill on a long (TAP) layover in Lisbon. We are young-ish, healthy and ready for anything! I was thinking of booking a private tour with Yellow Cab TT Tours? I read a positive review of them somewhere. We want this to be an easy day (as far as transport to and from the airport) and a fun and memorable one as well!
I'd start with a guide book that covers Lisbon. See whether there are specific sights in Lisbon you particularly want to see. A private tour may be a smart move with 4 people and so little time, but you will probably have more fun if the contents of the tour take into account the interests of your group. You might decide to spend part of the day on your own (Jeronimos Monastery/Belem? Gulbenkian Museum?) and then have a half-day drive-around tour by taxi. My philosophy is that there's no sense paying for a driver's time if there are places you want to go inside that will take a good bit of time. There's public transportation, plus regular taxis, to get you from Point A to Point B if that's all you need for part of the day.
Great ideas from acraven! I would also suggest to decide what sights you are interested in. I am not familiar with Yellow Cab Tours. We were in Lisbon a few years ago to join the RS Portugal Tour. If I had one day I might take a 3 hour guided walking tour like the "Best of Lisbon Walking Tour." I read that they end the tour with Tapa's and wine! www.lisbonriders.com. We used this company for a tour to Sintra. I have read excellent reviews on their walking tours. As mentioned up thread you could also add some other sights to visit on your own as well. I would not hesitate recommending Lisbon Riders. They offer both group or private tours.
Is that 12 hours total or 12 hours plus the time you need at the airport (2-3 hours) for your next flight? Also depends on the time of day you arrive. I enjoyed St. George´s castle, which also has great views of the city.
I highly recommend the Lisbon Insiders "Best of Lisbon" 3 hour walking tours suggested by Janis, providing it fits your schedule. It meets in Rossio plaza which is very easy to get to by metro from the airport if you want. It covers parts of Baixa, Chiado, castle area, and Alfama with stop for pasty and another for cheese/ham/bread and wine. The guide I had was very helpful in individual directions on how and when to get to other places anyone asked about to plan the rest of their day. It did not include Belem or (if I remember correctly) Bario Alto.