If I only had one sensory bin left, it would be water. It costs nothing, it needs no prep, and it is the only setup that works for my 3 year old and my 5 year old at exactly the same time.
The trick I learned the hard way is the towel. A folded old towel under the tub turns a flood into a wipe, and it means I can say yes instead of hovering.
Warm water is the second trick. Cold water gets abandoned in four minutes; warm soapy water with a sponge in it keeps them there.
And there always has to be a job — cars to wash, animals to rescue, boats to sail. Water alone is nice. Water with a mission is an hour.
Tip
Never leave a toddler alone with a tub of water, even a shallow one. In summer we take the whole thing to the balcony; in winter it sits on a towel on the kitchen floor and I get dinner started nearby.