One option is the Quick Shuttle from Vancouver to Seattle. You may not find it as efficient as other options, however. While the bus company is efficient, you can be at the mercy of Customs.  Buses must unload all the passengers and luggage and bring everything thru Customs and Immigration.  If multiple buses arrive at once delays are pretty common going or coming.

Plan accordingly, your trip may be delayed by several hours.

The Washington State Department of Transportation web site shows, graphically, the best and worst times for crossing the border, both north and south bound. It also breaks them down by season.  

You can also drive yourself in your own car or rental vehicle. Waits of up to 2 hrs are not uncommon in Summer.  Check with radio 1130 am for border traffic reports.  Border crossing info is also on another inside page called Trips to Vancouver and Victoria and on smartphone apps.

You also may find that returning to Canada is easier than it was to leave! For example, travelers that took Amtrak from Seattle to Vancouver have found Canadian Customs to be quick and friendly.

The scenic Amtrak Cascades train leaves Vancouver Pacific Central Station, daily at 0640 and 1745. The new (as of August 19, 2009) 0640 departure is a through train all the way to Oregon whereas the evening 1745 depature is only to Seattle.  For 2012 Amtrak has announced some new changes. First you clear customs in Vancouver so it should make the trip to Seattle a little faster as the train will no longer has to stop in Blaine.  Second there has been some talk of adding a midday train so check the schedule to see if and when this is implemented.  You should also highly consider using the Amtrak bus vs Quick Shuttle as it makes no stops and stops in downtown Seattle vs by the Space Needle like Quick Shuttle. 

Please see the hints on crossing the border on the Seattle inside pages.