J.L.
I have a friend who just booked thier stay down there...the cheapest would be staying in a cabin at the KOA at La Selva Beach....she booked two nights and wiht tickets to the aquarium was only $300. The KOA is very nice and has a great park, with a train ride and jumpy thing.
We go there every year for a Halloween Festival which books up a year in advance...each month they have some sort of family friendly celebration..so those weekends at the KOA might be a little more expensive....they usually have free smores by the campfire....they have a coffee/espresso wagon, etc...
What is near.....the aquarium is 30 minutes away south....santa cruz is 10 minutes away north.....la selva beach about 2 minutes.....you can also drive to Capitola..fun little town with shops but a beach too that you don't have to hike down rocks to get at.....there are forrests to explore and there is also the 17 mile drive of just oceanside.....there are some places you can park and go down to rocks and let the kids explore the little pools left by the tide to see the life/fish in them.
Monterrey has a board walk..but we have yet to get to that, since we spend so much time at the aquarium...4 plus hours...that we are so tired and ready to go back to KOA...the food at the aquarium is good too. The aquarium has lots of kid interactive stuff and they recently built one area, that is just for kids to play on.
We have yet to go ot Carmel, but I hear it is beautiful....we are extending our stay and going next year!
Have a great time!