Seattle's not as boring as some have claimed. It has it's hot spots and it's a pleasant place to visit. It does rain a lot but you should be safe around May. It's been real nice as of late, since the Jet Stream is swaying around California resulting in them getting our typical Mid-Winter weather(cloudy/rain). If your looking for the tourist attractions, it's not hard to miss the infamous, "Space Needle."
It's located in an area called, Seattle Center. There you'll find the EMP(Experience Music Project), that some have mentioned. If your into NBA Basketball, the Sonics Key Arena is also in this vicinity. Come May, Playoff time. Who knows, Conference Finals.
That area is called Queen Anne Hill. You should check it out. Has lots of great Cafe's and Restaurants. Yes, you will find lots of Starbucks shops during your stay. When your done there, take the Seattle Monorail(sky train) from the Seattle Center to the center of town.
If you want to know where everything happens around here, Downtown Seattle is definitely your best bet. It's where everything comes together. Very Lively. All sorts of places to go: Niketown, Westlake Center, Pacific Place, Pike Place Market(Fish Market), New Seattle Public Library(Glass exterior), Seattle Waterfront(Argosy Boat Tours, street entertainment), Gameworks Seattle(kind of played out), Cinerama(best movie theatre in the area. Episode III, anyone?). Metro is free in Downtown, btw.
Head up North on Pine Street and you'll land eventually in an area Called, Capital Hill. There, you'll hit Broadway Ave. You should find a Jimmy Hendrix statue right on the Ave. You can't miss it. Again, lots of fine shops and cafes. Some people say it looks very much like a European street area.
This is where you'll find all the stereotypes: Grunge, Hippies, Punks, Goths, indy groups, etc. Lots of nice clubs and bars to go to. Just be careful not to accidently go into a Gay one.
If your looking for the game scene there's not much left as far as Arcades go. They've been disappearing the last two years. Anyway, Gametown in Tukwila(10 min from Sea-Tac Airport) is one of last still standing. They have all the great SNK/Capcom Fighters. 2 & 4-slot MVS galore. Pretty Cheap too.
Uwajimaya/Kinokuniya(Japanese shop) is also located in the International district, Chinatown, if your into that stuff also.
Well, hope i've helped out. Enjoy your stay in the Emerald City come May.