There are lots of ways to accomplish that, depending on your interest and budget.

A ferry ride to Bainbridge Island is one easy way - walk on to the ferry, get off at Bainbridge Island for an ice cream or coffee in town, wander the shops and then hop on for a return ride. Do this one on a sunny afternoon for the best views of Seattle on the return trip.

Other possibilities include:

Riding the Ducks tour - their vehicles will take you into lake union for a peek at our houseboat community.

Rent a rowboat at the center for wooden boats (Lake Union)
Rent a canoe at the UW boat center (Lake Washington)
Take an argosy cruise through Lake Union and the Locks.

The underground tour in Pioneer Square is interesting if you haven't done it before.

The Museum of Flight at Boeing Field is good too, especially if you are interested in airplanes or history.

If you have any free time left (looks like you have a lot of stuff planned!), a walk around Greenlake is nice.

Enjoy your trip!