Excellent suggestions as far as gear stores are concerned. Just to reiterate, I think the best cluster are by REI. Feathered Friends is across the street and Outdoor and More is only a few blocks away.
As far as hikes are concerned, it depends a bit on how much time you want to spend and where you want to leave I-5 (on your way South). Regardless, I feel you should go into the mountains far enough to get into wilderness (often this is only a few more miles of driving than where many people hike). With that in mind, I recommend the following:
End of the Mount Baker Highway: Table Mountain or Ptarmigan Ridge. Ptarmigan Ridge probably won't be melted out, but it is fairly flat, so if you are comfortable on snow, this is a good bet. I think Table Mountain will be melted out. Even if neither are melted out, the view from the parking lot is rather spectacular and worth it. This drive is a bit further than my other suggestions (it is 60 miles from Bellingham) .
Lower Part of Mountain Loop Highway (outside of Everett or Marysville): Mount Dickerman is a very nice hike, with outstanding views and flowers, but plenty of people. http://tinyurl.com/2wtaa7
I-90, Snoqualmie Pass:
Snow Lake, Gem Lake, Wright Mountain -- Snow Lake is very popular, but if you keep going you can lose the crowds. Wright Mountain sits above Gem Lake and is officially a scramble (no official trail) but very easy (if not, then you probably are not on route).
Snoqualmie Peak -- This is a scramble. If you are comfortable doing scrambles and don't mind icing your knees after the day is done (it is about 3 miles and 3,000 feet up from the road), this is a very quick way to get excellent views. As scrambles go, this is an easy one -- there is a path, but not an official path. Park as for Snow Lake, but find the path about 20 feet or so south of the Snow Lake trail head.
Pacific Crest Trail to Kendall Cat Walk and Beyond -- This is probably the smoothest trail on the list (being the PCT). It takes a while to get up to views, but when it does it is very nice. So nice that you will have trouble deciding when to turn around. The Kendall Cat walk is a nice turn around spot, but since the walking is very nice from there, you might considering going on a bit (unless pressed for time).
A couple of web sites to consider as well:
www.wta.org -- Washington Trail Association -- This has great information such as trip reports and links to ranger stations. You should be able to get trail information directly from them or from the associated ranger station.
www.nwhikers.net -- This is a forum based site with plenty of information. You may have to join up to post a question (this would be handy if you can't find trail information based on my info).
A couple more things: You need to buy a trail park permit to hike. If your friend has one, borrow it. If not, I think you can buy a day pass at any of the hiking stores, a ranger station and at many of the trail heads. Then again, if you are driving a Canadian car, it may not matter if they ticket you.
For a map, you can try topozone (free) or buy one. If you buy one, I recommend Green Trails.
Here are some pictures I've taken of some of the places I mentioned:
Snoqualmie Peak: http://www.flickr.com/photos/rossbl/sets/72157594207362420/
Kendall Cat Walk: http://www.flickr.com/photos/rossbl/sets/72157594322453543/
I don't have a summer picture of the Table Mountain/Ptarmigan Ridge area, but this is a winter picture of the trailhead/road area: http://www.flickr.com/photos/rossbl/sets/72157594456870948/