No need to pay for any sightseeing tours in Seattle, unless you want to experience very specific things like the Underground tour. Downtown Seattle is pretty small, very walkable. We are getting more daylight now, sunset is around 8pm and it will extend to almost 10pm by June.
Avoid 3rd Ave for sure because that's the main hub for all buses and it's not a pleasant street to walk along. Go to Pike Place earlier in the day, get some donuts from Daily Dozen Doughnut as they are made fresh throughout the day. Don't go to the original Starbucks at Pike Place, line is always too long and there's nothing special at that location. If you are a coffee lover, plenty of great local coffee shops in downtown. My favorite in the area is Mr. West and Anchorhead and the are across the street from each other. They should be very close to your hotel since you're staying on 6th Ave.
I'm local so let me know if you would like any other recommendation.