Since you'll be there in April, the Carlsbad flower fields will be in full bloom and are amazing. You'll have to navigate around everyone there for the instagram pics, but it truly is gorgeous.
And I second the music museum. I took my son there and there was only one other family there. They have a lot of exhibits but then also have instruments you can actually pick up and play (or attempt playing). My son loved that he could try so many different instruments (and has been begging for an electronic drum kit since).