On one of my more recent finds at Discount School Supply was this pack of watercolor leaves. They aren't watercolor paper. I'd say that they are made of the same stuff as coffee filters. I've done something similar with the coffee filters and they work great.
I pulled out some spray bottles, small dishes with brushes, and the bingo bottles, along with red, yellow, and green liquid watercolors. The plastic sensory bin and a laminated cotton fabric "splat mat" helped contain any wayward spray. Finn is usually amenable to keeping his mess in the designated area, although many children would experience more freedom of play (read: spraying colored water everywhere!) if this activity was done outside.
Then we started spraying, brushing, and dabbing ... watching the paints creep together and the colors mix. We dried the leaves on newsprint on another table, then hung them behind our dining room table.
Some of you have asked where I get my ideas for these projects. While some of them (painting the sky) just came out of my head, most days I rely on the huge wealth of ideas online to put together a project. Here are some of my favorite places to look:
Play At Home Mom - Reggio-inspired play exploration.
Teacher Tom - Lots of tinkering and child-led activities.
Filth Wizardry - So many ideas, I don't know where to begin.
The Artful Parent - Inspirational art projects, many ideas for babies on up.
The Crafty Crow - Compiles crafty goodness from around the web.
Childhood 101 Playopedia - Another list of good ideas sorted by age and type of activity.
There are so many more, so please share your favorite placed to find ideas in the comments!