The first project of summer vacation this year was sugar + water.
The first segment was to make hummingbird food for the hummingbird feeder I picked up from Dollar General (technically, it was my kids' Mother's Day present to me). I found many online resources, that most matched this one. The ratio is one part white sugar to 4 parts recently-boiled water (presumably to sterilize it). We followed the admonitions not to add food coloring, nor to change the proportions. I instructed my kids to measure how much the feeder would hold (then water the plants with the excess), and to calculate how much sugar to add to that much water. We then set that aside to cool overnight. I had them sample 1:4 sugar water for reference, and I was informed that hummingbirds are lucky because that's tasty.
If you wanted to continue on the hummingbird theme, here's a collection of hummingbird resources (maps, books, crafts).
The second segment was to make rock candy. The directions varied with the website, from as high as 3:1 down to a moderate 5:2. I went with the super-saturated note from Science Bob and stopped at 1:1 (possibly too low) when the sugar took much, much longer to dissolve in boiling water. I dipped popsicle sticks (made of untreated wood) in the solution, then rolled them in sugar, put clothespins above the sugar level, and placed the sticks in small (to the tall and narrow side) jars to wait for cooling and crystals. Next time, we might add flavors!
And now we wait impatiently for sugar crystals to grow ...
One piece of trivia from my college Optics class is (unverified) that sugar water circularly polarizes light, so you might be able to add on further science ...
