SEA is usually very pleasant in summer. Lots of outdoorsy stuff, and things to do on and off the seafront. The space needle and the surrounding area (the Chihuly garden should appeal to both daughter and dad). The underground tour is fascinating, but she may be a tad young for that. However, it's nearly as far as San Diego (although it might be cheaper) - but as a 5-year-old and a short daddy you shouldn't be too uncomfortable even if you don't get upgraded
.