Quite a few small side trips you can take-needs more than 1 day

-Boeing Factory tour in Everette (open weekdays only)

These are not exactly on I-5, you have to drive on the side road but are easy to get to from I-5
-Mt Loop Scenic Byway to Big Four Ice caves (via the town of Granite Falls). Ice Caves are best viewed in late June early July and a whole valley of waterfalls within a short 1 mile easy hike
-Deception pass area and Whidbey Island off SR 20. Fort Ebbey State Park is another nice one with 360 degree view of snow capped mountains (on a clear day), Puget Sound, light house and old fort.
-The town of Anorcetes off SR 20 (Washington Park overlooking San Juan Islands)
-Take Hwy 20 East will get you into North Cascade NP area. If you don't have time to go to Cascade NP, get onto Baker Road, and you will uncover some hidden jewels, including two snow capped mountains reflected in a lake.
-Take the Chuckanut Drive exit off I-5 (it might say Hwy 11) and reconnect to I-5 in Bellingham. Beautiful drive overlooking the San Juans along Puget Sound the whole way.
-In Bellingham, take Hwy 542 east and you can go all the way to Picture Lake and Artist Point, one of the most photographed areas in North America.
-On your way back from Artist point, do not come back to I-5. Instead you can take the road going North to enter Canada, this entrance is much less crowded than the I-5 entrance.

This list can easily take you a few days.... instead of one day!