I'm new the DISboards, as a Californian planning their first trip to WDW. So I saw your thread and thought I'd offer my thoughts, since I grew up in Carlsbad and moved to the Bay Area last year when I graduated from college.
First off, I definitely recommend you eat at an In-n-Out during your trip! There's one in Carlsbad off of I-5 on the Palomar Airport Road exit. Best fast food EVER.
Other than Ruby's on the Pier in Oceanside, there's not much family entertainment downtown besides the surf museum (I've never been, so I'm not sure what to say about it). Some areas of Oceanside are slightly sketchy, but you should be OK on the pier.
Carlsbad has a cute downtown area with antique shops and restaurants. It feels much safer to be walking around after dark than Oceanside. Besides late night coffee shops like Vinaka's, Fidel's offers Mexican food and Neiman's offers American fare in a neat historical building. I would definitely recommend Pizza Port (a local brewery with pretty good pizza or Vigilucci's, a more expensive, but great, Italian restaurant on State Street). You can also walk on the beach and its seawall after the sun goes down with the rest of the locals. Carlsbad has some great beaches. Ponto beach (at the very South end of C-bad) is usually less crowded and has free parking.
Next to
Legoland are the flower fields, which are lovely in the spring. If you don't want to pay for a tour, you can get a pretty good view of them when you shop in the nearby Carlsbad Company Stores. (Which have some really good outlet stores, like Gap and Coach...) There's also a Ruby's in the Company Stores that features displays of vintage toys.
The problem with North County San Diego is that you have to drive a bit to get to some other attractions. The next town South from Carlsbad, Encinitas, has a REALLY neat, laid-back downtown area located along Pacific Coast Highway. I spent most of my teenage years in the vintage shops and coffee houses down there. For kids, Moonlight Beach in Encinitas is very family friendly. The next town south, Del Mar, is full of drool-worthy million dollar homes, but also the Del Mar race track. If you're going in the summer, it's always fun to go watch the horses race.
If you don't want to drive into San Diego, I'd recommend riding the coaster to either Old Town or Downtown San Diego. You can hop on at the Oceanside station, and the ride is beautiful - you'll get to see some of the best stretches of the coast without being trapped in gridlock on the 5. Old Town is a neat historical state park that includes one of the most haunted places in the U.S., the Whaley House (I loved this place as a kid, but alas, never saw a ghost).
If you ride down to downtown SD, you'll have to walk a good deal to get to the best attractions. For instance, you really have to drive to SD to go to Balboa Park, but it's definitely worth it. There are so many museums and the zoo is, like it says, world-famous. Traffic is the worst going North on I-5 between 3 pm - 7 pm, if that helps at all. In good traffic you should be able to make it down to SD and back to O-side around 45-50 minutes.
I'm sure that's probably along the same lines of what the tour books recommend, but I just wanted to share.
Hope you have a fun trip!
