Here is another great idea from Fun at Home with Kids! I filled a bucket with some soapy water, added a few different sponges, a cloth, a brush, and voila, an instant hit.
Miko enjoyed squeezing the water out of the sponges. He also tasted the sponges quite a bit, I was surprised the soap didn't' deter him. :)
His expression slowly turned from a look of quiet concentration into a full on exuberant smile.
I think the brush was his favourite