I am a 26 year old (well will be then) and am thinking of going to Legoland while I am in California. Is there a lot for adults to do there. I am fascinated in seeing anything made out of Lego so that would be something that I would want to see.
Well, most people are going to tell you it's just for younger than 12's.
I'm going to say - do it if you're interested in Lego. If you don't go you won't know & willl wish you had gone. I haven't been in a couple of years but have gone likely 6-8 times. I loved it. It's not the hectic pace of DL. It's much more relaxed. The grounds are beautiful, I'm in Canada and we don't have all those pretty flowers.
Miniland is incredible, I'm sure you can look on their website to see some pictures of it.
While you likely won't spend a full 12 hours there short estimate is likely 4 hours but that will depend on how casual you make it. You can usually find coupons for a few dollars off adult tickerts.
Go back further in this section and you'll find quite a few threads about LL.