Ice excavating is one of our FAVORITE outdoor activities! There are so many different ways you could take this as far as themes go.
Fill a container (metal cake pans work GREAT) with small toys and/or objects. Fill with water and freeze. A 3" cake pan takes just a couple of hours to freeze completely.
Once solid, turn upside down and run warm water on the back of the container until the block of ice comes loose.
Use small tools (mini screw drivers, small hammer) to crack the ice open. Once the kids get a nice crack going, the rest breaks apart really easily and they have no problem retrieving or "saving" the toys.
We've done this activity countless times and it's always fun to surprise the kids with this in the summer when friends come over.
Easy, inexpensive, hands-on... go ahead and try it!