We love the Bay Area Discovery Museum. They are very much geared to the little ones, with simple, easy-to-do hands-on science and lots of room to run around. I'll share some of J's favorite things.
Some various sizes of threaded steel rods and washers was the first eye-catcher of the day. They had tire with rods threaded rods sticking out of them. Varying the amount of washers and the size of the washers and rods, J got to play with different sounds!
I don't think J realized how much fun I was having with the plastic cylinders of water. Silly faces weren't the point of the frog blowing bubble exhibit, but I sure had a lot of fun with it. J enjoyed seeing the pictures on the back of the camera. The water acts as a magnifier in the cylindrical tube.
Ah! J's split in half! See if you could split your child.
When all else fails, FunNoodles always make for fun science. We often use it as a telescope, "I see you!" and as a microphone, "Do do do!"
And, if you want to go Biology, find/build a nest and teach your kid about hatching eggs. It's almost like a playpen.