I think we all have fond memories of bathtub fun, or water tables at various preschools and daycares. But what if you don't have an actual water table at your library? I know I don't.
I improvised this for the very first time we ever did "Senses," and it was such a treat that we bring it back every time that theme returns. There are all sorts of STEM experiences in this simple activity. Buoyancy, the behavior of liquid, the holding capacity of a tiny Tupperware container . . . possible experiments are endless.
That's a Rubbermaid tub, about six inches deep, with a lid. The lid is awfully important, since we have everything set up in the storytime room beforehand, and kids are inevitably curious. The toys inside are simple things we found around the library. Foam numbers or letters will float, and also stick to the plastic sides. Tiny cups lead to floating and pouring and sinking and all manner of fun. Try out all sorts of things - we did!
I filled the tub about halfway and set it on a child-level table with a plastic table cloth underneath. You can get about four to six kids around a table, depending on how well they play with others. Then at the end of storytime, we removed the lids and let the kids get splashy.
When everyone was finally dragged away, we removed the toys, dumped the water, then put tub and toys out in the sun to dry. Our arid climate and the presence of a staff patio helped with this; everything was ready to be put away within an hour or two.
One caveat: it's a great summer activity, not so marvelous in February, as the kids will get water all down their fronts.