We are going to be in Lisbon for a day and 1/2 upon conclusion of a Windstar cruise. What is there to do?
You'd probably find better guidance on the Portugal Forum, as this is the Spain Forum.
Sintra, Belem, Rick's three city walks in his guide book, Gulbenkian Museum. Great city, easy to walk but some steeeeep hills. Check out Rick's guide book or for a quickie, go to the daily itinerary of ETBD's Portugal tour and check the activities listed for Lisbon.