I am a frequent flyer at Legoland, and there really aren't many hidden things...pretty limited park...lots of fun, but not much you would miss as a casual attender. In Fun Town, there is a silly simple walk through exhibit, called "Adventurers Club". It is easily overlooked. My daughter who is now 6 would want to go through it 4 times in a row when she was younger. Now we can get away with just once.
There are a few/many places to stop and play with legos in the park which can give adults a chance to sit and just "be", big one in Knight's Kingdom , one inside a snack place in Fun Town and in Play Town one geared to little kids inside a giant interactive playground others that I am not thinking of at the moment.
My best tip if you are not totally anti-sweets is to stop at the candy shop on the way out...by stroller rentals and buy $1 of bulk candy. Makes kids happy to leave...and not a lot of money for a souvenier!! Works for me, but we have been there a lot.
Have fun