We don't have Sea World passes, but we do have passes to the zoo as well as the Children's Museum downtown. My boys love the Children's Museum, and we hung out there quite a bit this past summer as it's indoors and air-conditioned! They have a super cute pretend HEB downstairs, and my 14 month old uses the kiddie shopping carts as a push toy. I think he was maybe right around 1 year when we started going there.
The zoo is fun too, but we haven't been much lately. I enjoy it more during cooler weather. We did go last Monday though, and both my 2.5 year old and 1 year old loved it. They also have a little playhouse in the toddler area that is fun for the little ones.
I think the Zoo would be a better bet for the next year...and then Sea World. You can do a family pass for the zoo and take a grandparent or anyone else really...they have a fun area for little ones. My stepson loved the zoo when he was little.
The hard part about either place is that if we have another summer like this one...you won't want to go to either place. We had Fiesta Texas passes this summer (courtesy of my dad) and went one time. We are planning on going a bit in the fall and winter before they close...but it was a bummer it was so darn hot all summer and kept us from even wanting to go!
Either place will be fun!!