One of my most successful activities for my preschooler has to be water play. I hosted a 4 week Play-based art group as part of my master's capstone. This activity held all of the children's attention for the majority of the 45 minute class.
The only thing you really need for water play is a large bin or plastic storage box. I used shallow bins from Ikea and placed them on wood crates to elevate them a little bit. I them placed a variety of tools like mesh strainers, spoons, fuzzy balls, sparkles, tissue paper, and paint. The kids loved using the strainerstrainers to get the fuzzy balls.
Large Bin or storage box
Fill the bin with water either outside or inside with paper or a rug beneath it. I noticed our tile got rather slippery once the kids started splashing around. Then place a small box filled with the tools and mediums. Then sit back and watch the children play and explore. They will quickly find out how strainers work and will enjoy the new colors the water turns.