Here are some routes we've done:
Fairhaven -> Chuckanut Drive -> Edison -> Bow Hill -> Colony Drive -> Lake Samish -> Fairhaven (40+ mile loop)
Chuckanut's dramatically beautiful, if you don't mind being trapped between cars and cliffs. Skagit County also has lots of quiet beauty (and birds), particularly in the Western portions. There are 2 bakeries in the tiny town of Edison, which make it a great rest stop. The ride back past Lake Samish isn't very scenic (particularly on the Eastern shore), until you get to Old Samish road or whatever it's called, the one that goes past Pine and Cedar trailhead. Then it's pretty again, and there's a relief that the hills are almost over!
We sometimes continue to Bay View state park, or down past Hwy 20 to the tulip fields when it's tulip festival season (April or May). Also, a couple times we've come back via Alger, Lake Whatcom, and Sudden Valley, which adds another 20 miles, lots of hills, and Lakeway (traffic).
Bellingham -> Mount Baker Hwy -> Glacier (35+ miles one way)
This isn't a loop, but it takes you past some nice mountain foothill scenery. I like the horse ranches and cows. You get lots of glimpses of mountains. The Glacier ranger station (near mile 35 of Mount Baker Highway) is a perfect rest stop, since it has bathrooms, places to eat outside, and a nice vibe.
If you're feeling ambitious, you can push further up Mount Baker Highway, or you can return via the northern loop around Sumas Mountain (through Sumas and then Nooksack and Everson). Or you can veer south at mile 14 or 15 and return to Bellingham via the eastern shore of Lake Whatcom (through Sudden Valley).
Bellingham -> Ferndale -> Blaine -> Birch Bay -> Lummi -> Bellingham (~60 mile loop)
The fastest way to get to Blaine is via Portal Drive, which parallels I-5; however, the scenery is lousy and there are lots of cars. To enjoy yourself, you need to take the side roads, which are longer (since they are not on the diagonal to Blaine) but have more cows, horses, llamas, bison, etc. Blaine itself is boring, except for the Peace Arch (see note below). The ride down to Birch Bay is nice, especially if you take Drayton Harbor (I think it's called). I usually go down to scenic Birth Bay State Park, where there are bathrooms and a place to swim (if you're into that). After that I head towards Kickerville, Slater, then Lummi, where I get on Marine Drive, which is the long final stretch back to Bellingham.
NOTE: if you intend to bike past the Peach Arch (even just within the park area), be prepared to show proof of citizenship to the Canadian customs folks. They are now saying that we should check in with Canadian customs even if all we do is walk under the Peach Arch and back. Bikers are apparently regarded as a more serious "threat" than pedestrians. So, if you don't have your passport or birth certificate along with you (who does???), consider either skipping the Peace Arch, or parking your bike on the US side before walking over to the Canadian side. If you do get harassed by customs officials, don't bother arguing or reasoning with them because their job is to treat you as guilty until proven innocent, with the excuse that you have trespassed on "foreign soil", and are completely at their mercy, with no "rights" left. They'll search your panniers or backpack, make you sit for a long time, and then, when they finally release you, they will tell you that you now need to go to the US Customs and do the whole thing all over again. So, if you don't like the idea of losing an hour or two of daylight while subjecting yourself to the arbitrariness of post-911 customs officials, then I advise you to stay on the US side of the Peace Arch, at least until things cool down again. Forget about those benches up on the Canadian side. They just aren't worth it!