My recommendation is not to even bother with an umbrella, wear comfortable waterproof shoes, and hide your hair beneath a funky hat of some sort that will handle getting a little wet.
Whatever you do, don’t let the rain spoil your good time. Rain is a fact of life in Seattle and most of the locals are quite accustomed to it. If it really does bother you, you may wish to plan an alternate indoor activity for every outdoor activity you are considering. The good news is that there are plenty of both to choose from so you really shouldn’t ever run out of things to do while on vacation here.
If all else fails, there is plenty of great shopping in and around Seattle that will get you off the streets and into the stores. While I love shopping, unless we are talking small local stores, I can do that for the most part at home. I love visiting the local shops and galleries and buying pieces of pottery or art by local artists, other than that, I have shopping malls and chain stores at home and have rarely found something new and rare at one of these stores that I couldn’t have found at home or online. But some of us are addicted to shopping in any form and really; there is no shame in that. I have a serious weakness for shoes so I can completely relate on some level.
Other than shopping the local shops and stores, there are a few other indoor activities I recommend checking out. First of all there is the Experience Music Project, The Seattle Children’s Museum (surprisingly fun for all ages), the Pacific Science Center, the Spirit of Washington Dinner train, Redhook Brewery tours, SeaBear Smokehouse tours, and a ride up the Space Needle which is a must for any trip to Seattle (I suggest getting dinner elsewhere and simply taking the ride up if cost might be a factor).
In addition to these you can take the Boeing tour and see how these huge planes are made or trek to the Museum of Flight, which consistently ranks among the top museums of this type in the country. You can take in a movie at the IMAX Theater or enjoy the Seattle Aquarium, which will allow you to take a different point of view on the things that live in Puget Sound.
If food is something that interests you, there is plenty of great food to be had throughout the Seattle area, but nowhere offers the selection that can be found at Center House in Seattle Center. While you are there, be sure to check out the Northwest Craft Center Gallery for a nice selection of Native American and other artistry. Other great places for food and dining are the Pike Place Market where you can watch your meal prepared right before your very eyes. You can also elect to take a class or two in gourmet cooking through PCC Cooks if this is a skill you’d like to bring home with you.
As I’ve said, there are plenty of things to do indoors in Seattle if you need alternatives to those rainy day blahs. Of course, you could always make the best of the situation and try singing in the rain during one of the many outdoor activities that abound in this truly unique American city. Only you can decide best how to handle your trip and the weather that may intrude. But a vacation is supposed to be fun so it is best to have a backup plan that you find agreeable rather than spending precious vacation moments grumbling about the weather.