Seattle Center is trafficky and somewhat chopped up by SR 99 and appendages, which has a tendency to create under- or overpasses that makes walking a pain.

The Denny Park area is very trafficky, no attractions to speak of, and there are a couple of streets/alleys that can be a little iffy at late night. If you stayed in that area I'd get a car, personally.

Have a look at 3 properties around South Lake Union, especially the Residence Inn (Marriott) or the Silver Cloud Inn, both of which have easy access to the South Lake Union Trolley (not its official name but everyone likes to call it the SLUT) that goes to the retail center. The third is the Courtyard (also Marriott) which is a couple of blocks from the SLUT stop but also (usually) decently priced.

Silver Cloud (www.scinns.com) also have properties in the Broadway district (very lively, lots of restaurants, bars, rainbow flags) and across the street from Safeco Field, very convenient for baseball games if that's your thing.

if your cruise departs Pier 66 (NCL, transients) the waterfront Marriott and Edgewater (both expensive) have the advantage of being walkable, so you should mentally deduct a couple of $20 cab rides from the hotel bill and reevaluate your budget.

Remember May is still the wet season hereabouts, so being close to the free bus zone downtown would be a plus if water comes from the sky.