Apparently the gum wall has made it into guidebooks. When I lived 200 feet down Post Alley in the 90s, it was a minor thing (in size and popularity) with occasional tourists. Now it seems to have a mini-cluster all the time.
As for the "summer crush," the Market, the waterfront, the Seattle Center, and the Retail District do get a lot busier in the summer. The Market gets particularly nuts. I pride myself in being able to get through any crowd quickly, but even I'd need 20-30 minutes to walk the three block main hallway right now.
^ Pike Market and the Westlake retail district were definitely hot spots constantly bustling with activity.
I was also very impressed with the quality of new buildings in Capitol Hill and Belltown with their integration of engaging forms of retail usages. Great pedestrian streetscape interaction.