Since it is LEGO week - let's have some STEM fun with them too. What kind of vehicle will make for your zip line? What goes the fastest? Take into account the weight of your design. And what about the friction of the vehicle on the line itself? There are a lot of variables to look at. 

So it makes cool art. But here is my 2 cents - use this to explore color mixing. Use only blue, red and yellow food coloring. Let the kids see what happens when the sale absorbs more than 1 color.

You need 2 cups, water, cotton string,  and some tape/paperclips to hold the string onto the cup
Wet the string first and attach each end onto a cup. Fill one cup with some water and with the right angle you can make the water travel like magic down the string into the other glass.
WHY? Gravity, adhesion  and cohesion. Watch his explanation and you will understand!

You need an egg, a glass and white vinegar.
Put the raw egg in the glass and cover with white vinegar. WAIT 3 days (I think we have got the time). gently rinse in tap water. Your egg is now translucent and BOUNCY!
WHY?
If you soak an egg in vinegar the eggshell will absorb the acid and break down, or dissolve. The calcium carbonate will become carbon dioxide gas, which will go into the air. What is left is the soft tissue that lined the inside of the eggshell.

First - put a bunch of ice in a bowl and give the kids a string and tell them to try to fish for the ice. 
Problems right?
Now give them some salt or call it "magic crystals" to sprinkle on the ice. What happens to the ice? Can they figure out what to do next?
Why?
Laying the string across the ice cube where the salt is makes the ice melt and then since the water is still on top of the ice cube it refreezes and in that process the string sticks to the ice cube.  Talk about why they use salt on roads in the winter. Or look up how to use salt to make ice cream. 

Chromatography: All you need is markers, water and coffee filter. You can even do this by cutting one coffee filter into strips and putting a dot/line at the bottom where you will add the water. Let the coffee filter draw the water through the maker and it pulls apart colors.
What is Chromatography? The separation of a mixture by passing it through a solution.
In this case it is marker pigment and water. Did you know there are hidden colors in your markers? try different ones and see what happens.

You need a parents help to make the super saturated sugar solution on the stove top.  It makes a very syrupy solution.  Then add food coloring . You will need some tall glasses and wooden skewers (if you don't have that use string tied to a popsicle stick over the top of the glass). It does take a few days to see the crystals start to appear.
Why?
The concentration of the sugar solution you made increases gradually as the water from the glass evaporates. As the sugar molecules are moving around in the solution they collide with other sugar molecules and stick together (that's why you started the stick with a dip in sugar). The longer the water evaporates the bigger your rock candy.

Or paper helicopters. Whatever you call them- they are fun to make and even funner (is that even a word) to fly!
You need a piece of paper, scissors and a paperclip or some tape.
Cut the paper like in the video where the top piece is in half and the bottom is cut in thirds.  Fold the bottom thirds towards the middle and tape or use a paperclip at the bottom to hold them together. On the top, fold the 2 blades in opposite directions so it looks like a helicopter blade. Now just toss it in the air.
Do they go faster it you add more weight to the bottom? What if the blades were bigger? Try different combinations to see what happens.