Only really 2 lakes up there to choose from - Whatcom and Samish.
Samish = Better water, less traffic. Just south of Bham
Whatcom = Big lake that gets blown out a lot and gets tons of traffic but luckily there are a decent amount of coves to claim that stay relatively flat. The launch at Blodel park can get REALLY crowded on hot days so be warned. Get there early to avoid the crowds. Also last I heard, the sheriff's department or the fish and game department was charging $$$ to inspect each boat that get launched. Looking for invastive species and what not. Luckily my buddy lives out in sudden valley so we launch out there and just stay down in the south end of the lake. You need a gate key to use the launch though.
One more thing about Blodel park...if you get there a little late to launch on a hot day and they have you park out in the field, make sure you be very careful about how/where you park. Last time I launched there I had to park in the field and almost couldn't get out because the whole area turned into a giant cluster f*ck. The "Parking Attendants" resonsible for guiding people to park somewhat organized did an aweful job....so again, be warned.
So yeah...my vote would be Samish.
Also, Bow Lake is a private ski lake located about 20 minutes south of Bham. Pretty sure they do lessons and private sessions etc. That could make for a real nice weekend!
Have fun, and if you see a white Epic 23v out there come over and say hello