There are some polymer crystals which can absorb 100x their weight in water and slowly release it. This is ideal for putting around plant roots in the summertime and these are sold in gardening centers all over, but they can be expensive. A cheap alternative is the same thing in toy form, called Orbeez. These also make a nice decoration in a clear vase with cut flowers. I bought some and they work as advertised. Sometimes you can buy these at the Dollar Tree already bloated with water. You might get 50 for $1usd.
Example: https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2334524.m570.l1313.TR12.TRC2.A0.H0.Xorbeez.TRS0&_nkw=orbeez&_sacat=3213&LH_TitleDesc=0&_osacat=3213&_odkw=water+polymer+gel&LH_BIN=1&LH_TitleDesc=0
These might also be called "aqua beads", "polymer beads", "hydrogel", or other names.
Here's an Aliexpress search: https://www.aliexpress.com/wholesale?catId=0&initiative_id=SB_20190702044601&SearchText=orbeez
1000 tiny 1mm dry beads is about $1usd, but they expand to about 12mm. 10,000 dry pieces are about $2usd. Some guy on Youtube bought millions of these and tossed them in his pool, then jumped in it! I guess several people did this. https://www.youtube.com/results?search_query=orbeez+in+pool
Example: https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2334524.m570.l1313.TR12.TRC2.A0.H0.Xorbeez.TRS0&_nkw=orbeez&_sacat=3213&LH_TitleDesc=0&_osacat=3213&_odkw=water+polymer+gel&LH_BIN=1&LH_TitleDesc=0
These might also be called "aqua beads", "polymer beads", "hydrogel", or other names.
Here's an Aliexpress search: https://www.aliexpress.com/wholesale?catId=0&initiative_id=SB_20190702044601&SearchText=orbeez
1000 tiny 1mm dry beads is about $1usd, but they expand to about 12mm. 10,000 dry pieces are about $2usd. Some guy on Youtube bought millions of these and tossed them in his pool, then jumped in it! I guess several people did this. https://www.youtube.com/results?search_query=orbeez+in+pool