Some sort of at-a-glance app for major events which will impact traffic (sporting events, parades, maybe even planned protests and school breaks). Perhaps even events at Key Arena/Seattle Center. If feasible, road closures and suggested re-routes (i.e. Broad Street, SLU changes b/c of Mercer construction work).
Your family's in town and you have nothing planned. App would allow you to quickly access local museums, parks, public art and events based on your current location or an address you enter.