Traveling between JFK and Zurich in May, we have the possibility of scheduling a 8-9 hour layover in Lisbon each way. I see the airport is located right next to the city, and the Moorish Quarter (for example) is less than an hour away by public transit.
I'd figure with public transport to/from plus a safe two hour buffer for security, we would still have a solid 4-5 hours in town. Given the opportunity of two separate 4-5 hour excursions, are there any Lisbon "must sees?"