Something that I think is fun but others might find boring: Fill a clear glass with water, drop in a drop of food coloring, and watch what happens. Repeat.
Or: Fill a bowl with water, grab a straw, drop little drops of oil onto the water's surface, and have fun (push the drops together, try to separate them, capture oil in straw and expel it under surface, etc.).
Other things to mix with water: honey (dissolve in water before adding food coloring and note the differences from plain water), liquid soap, you get the idea...
Oh, one that was even boring for me: Mix up water and oil very well and see how long it takes to separate itself. I left the glass in the window sill for this one. A strange thing happens sometimes - drops of oil will stick to the side of the glass and not move forever*! I don't know how this happens, but it also happened to someone I met at a party who had some kind of liquid-like material in a glass container. I don't recall what the stuff was, but that stuff was cool. He was studying Brownian motion if anyone wants to look it up.