There's plenty to do for little ones...not to worry. You'll do more together as a family than not! The list of what you can't do is a lot smaller than what youc an do!
One trip one of my children was 8 months, and on another trip one was 10 months -- they could do almost everything we did with our older daughter! The rides with height restricitions are too rough and a 16 month old wouldn't be allowed on anyway. Your five year old might not want to do them either. But so much is geered toward the whole family. Your littlest one will love it! And for the few things that your 5 year old wants to do but the little one can't (or is a sleep), you can just split up. Sometimes, I'd walk througha shop with my little one in the stroller while DH took our older daughter on something. Not a problem at all....
Enjoy!