Just follow the PCH. There's bike paths for pretty much all of it. You can go: the 101 -> Torrey Pines -> UCSD Campus -> Gilman -> Rose Canyon bike path -> Mission Bay -> back on PCH and you'll end up at the train station in downtown San Diego.
Can't remember the exact distance, but it's probably 30-35 miles one way, mostly flat except for Torrey. It's a lot of people's main weekend ride so you'd have lots of company.
Alternatively, you could take PCH south a bit from Oceanside and turn east to do something like Elfin Forest, SDBC, or Del Dios. A bit more miles and climbing, but way more scenic.
I mainly putter around 'Dido, but Revolution Bike Shop in Solana Beach recently did this 40 mile ride:
http://www.mapmyride.com/routes/view/60002014
I drive Del Dios every day, and the stretch along Hodges is awesome.