Both of our kids love to play in the water. Dalton gets so excited about "Baf!" and though we sometimes can't get Maggie in the bathtub, we sometimes can't get her out either. We got them an inflatable baby pool for Easter and they've spent hours playing in it out on the balcony.
Of all the sensory boxes we've done, the water was the biggest hit. Maggie loved playing in it, and the water was super easy to clean up. We did it during Dalton's nap time - big girls only. I laid down a towel underneath our big plastic tub. A couple inches of water in our tub, and some scooping and pouring toys were all she needed.
In the morning we made some colored ice cubes. We used our mini ice cube trays the first time.
And the second time we ventured into making bigger ice cubes using a muffin tin.
With some frozen dinosaurs for good measure. I recommend not using much food coloring. As you can see, no matter how much you use, the color will be much lighter when frozen, and it will dye the water a lot.
The kids also got some water/sand toys with their baby pool, so we added a few of those, and some seashells and rocks, fresh from the beach.
Linked at It's Playtime!