I was just in the castle one last week. In the lobby there's a free build area but it is pretty boring. Also a kid puked next to it while we were there, so we left after that!
Yikes. We did go to the regular hotel (which has a castle inside for the kids to run around). The buffet at BRICKS was fine for the money, but the blocks to play with in the open area were super boring - even my
Lego Lover (the five year old) was not interested in them until I said you can keep what you build to play with in the car

(although I had to force myself Not to think about how many kid germs were on those blocks by the time we got them). On a positive note, we did the restaurant instead of the park to save money and walking, for a five year old, that was enough (drive in slow and they can see lots of life size lego characters on the driveway) and the older siblings were much happier not to have to do
Legoland park. Parking for the buffet at the Legoland Hotel was $35 (yikes), and we were able to book the buffet time online.
The next day, we did the
Safari Park Zoo (cheaper, although you need to add another $25 each for a couple of rides to your entrance fee, $6 carousel and $18 balloon ride in our case, there are some other pay-to-do things also) - the Zoo was SUPER NICE to walk, with wide paths and a lot of shade, it reminded me of the clean and relaxing feeling of
Disneyland when it's less crowded. Rope Drop at the park is like 9am and if you get there then the parking is nice and close. If you do the Zoo, check out an Annual Membership for one person - some come with free parking and passes for others. With a local resident senior pass, we got the price down a bit doing that, but the math may not work for everyone.
Note there are two Zoos in San Diego, one more traditional located further south, the Safari Park in
north San Diego county has a tram ride of a huge savanna with a Lot of cool animals including giraffes included for free. Definitely the best part of the park. Parking at the Safari Zoo was $20. It's just under a two hour drive from Disneyland (90 miles), and inland - so would be HOT in the Summer, but you can stop enroute to see over the ocean, and throw food (we tossed a couple whole wheat ritz crackers) if you have some something appropriate for the seagulls, from a lookout right off the freeway too.