Hi. I have a couple of ideas too (from my days of teaching pre-school) I hope it's ok to post them!
If you're ok with your child using marbles this is a lot of fun. You'll need paint, paper, a bowl for paint, a high sided pan, some kind of soap and marbles. Mix some paint and a little dish soap in a bowl, drop in the marbles and stir them around so they are covered in paint. Line your pan with paper. Drop the marbles in with a spoon and have the child tip the pan back and forth to get the marbles moving across the paper. It's fun to use more than one color of paint so they can see how they combine to make new colors.
Car and truck painting
You'll need a long sheet of paper, like from a roll of butcher paper, paint, soap and some cars and trucks or whatever has wheels. It's really neat when you have some that have a texture to the wheels. Mix the paint and soap on a plate, roll the vehicles through the paint and then on the paper. By adding the soap, it makes it really easy to get the paint off the cars. Double the fun by giving the kids a tub of soapy water to wash the cars when they're done.
Washable paint would work too, but they didn't have that way back when! If you are using tempra paint though, adding the soap still helps with clean up and it also stretches the paint a bit so you're saving a little $$.
Wax paper butterflies and flowers
This one is a bit involved and requires an iron so a lot of adult supervision is required!
Have an adult shave some crayons in different colors, we used scissors to do this. Take a big piece of waxed paper and fold it in half, unfold it and let the kids sprinkle the different colored shavings on the paper. Fold the top piece over and have an adult iron it until the shavings have melted. When it's dry cut them into various shapes. You can then use construction paper to add details liek eyes and antenaes if you'd like. They look really pretty when taped up to a window and the light can shine through them.
This is a slightly different version on the tie dyed disks posted earlier.
Mix food coloring and water and use eyedroppers to squeeze drops onto the coffe filters. The colors blend beautifully and then they can be made into flowers!
Use your old paper towel tubes to make "binoculars" for the nature walks mentioned earlier. Have the child color or paint the tube, cut it in half and glue the sides together. (You can cut the tube in forths if the half is too big)
Using a paper punch, punch holes on the outside of each tube and thread with yarn so they can hang from the child's neck while you're out for your walk.
I have tons more ideas, but my messy house and impending MIL visit need immediate attention!