First, we have pearlized green glue turned into green slime that looks like, well, a bodily fluid that comes from the nose, which is the boys' favorite.
I also found red and pink gel glue that became lava slime. The plastic dinosaurs find it much more pleasant than real lava.
The last thing we tried was a combination of clear glitter glue and blue gel glue. It's more blue than clear and the glitter makes it really shimmery. The girls seem to like that one the best.
And, it's great for covering toys and finding them, wadding up and blowing into with a straw to blow "bubbles" and letting settle down your arm, just for the eek effect.
It's also a great science experiment, if you want to explain the science behind polymers. Here's a nice description compliments of www.science-class.net:
A POLYMER is unique because it has qualities of both a solid and a liquid. It can take the shape of its containers like a liquid does, yet you can hold it in your hand and pick it up like a solid. Solid molecules are tight together, liquid molecules spread out and break apart (drops) POLYMER molecules CHAIN themselves together (they can stretch and bend like chains) and that makes them special. Jell-O, rubber bands, plastic soda bottles, sneaker soles, even gum are all forms of polymers.
We also let them make and play with OOBLECK which is 1 part cornstarch to 1 part water. You can add food color as you like. Adjust the proportions and you have sidewalk paint when you add more water.
Oh, and don't be surprised if your older kids get in on the fun. My 12 year old daughter can't stay way from either substance!