This is called "Frozen Fun" and boy was it!! It takes some planning ahead - but the boys enjoyed the set up as much as the end activity. And it was basically free, since it uses water and found items! The original activity froze things in plastic bins - but I don't have any of those that aren't full of something I didn't want to dump out. So we used gallon size zip locks, and that worked out great.
So...when I was at the very beginning of my "Nope. No BEE-BEE shows. Play and have fun, boys" initiative, there was a lot of complaining, whining, asking repeatedly for "Gabba Gabba Gabba".
I quickly realized that I needed to do something with the boys, and plan things to keep them occupied (I did actually know how to do this at one point. It's like all the change sucked the problem solver out of me!) So after refreshing myself on what "Frozen Fun" required, I gave both boys a gallon zip. We went through the bins and baskets of toys in our living room and looked for small items. I gave them each a 1/4c measure and told them to find things that fit inside that. I still had to fish out some GIANT things, but it mostly worked. We had a LOT MORE little things laying around than I realized.
After they had enough in their bags, I filled them with water, put them on my large rimmed cookie sheet, and stuck them in our deep freezer. That thing is crazy cold (we have to microwave ice cream before we can scoop it), but it still took a while to freeze all that water. We tried taking them out after a couple of hours (maybe 4?), but they were still liquid in the middle (duh.) So wait several days...
Then, on an afternoon when you're kids are antsy and needing entertainment... Get them out, tear or cut off the bag, place it in a bowl or bin (we didn't want to lose the little toys to our deck), and let your kids go to town.
We'll be doing this again. I wonder about coloring the water, so they can't see the toys they're digging out as well - it would be like a treasure hunt!