My girls love Oobleck and we've made it a couple of times in the past. Bug has been asking me to make it again so I whipped up a batch and they took it outside and played with it until it fell apart.
1 cup water
1.5 cups cornstarch (more if needed)
And that's it! We added some food coloring to ours to make it pretty but it's not necessary to enjoy it.
I made them take the bowl outside and told them they could do whatever they wanted with it. Needless to say they both were practically covered by the time they finished.
They asked to make it again the next weekend and this time we made a pretty green color.
The more you play with this the drier it gets so you might want to have a cup of water handy to rehydrate it occasionally. My girls enjoyed using it as chalk when it started to dry out.
They made so much of this that we ran out of cornstarch!