Our bath letters are too big to use for this so I used our magnetic fridge magnet ones. Find a small (plastic) toy and spell it out using the letters. Put it all into a tuppaware, fill with water and stick it in the freezer overnight.
The next night, I took the ice blocks out of the tuppaware and gave one to each of them. They loved the feel of the cold ice, and enjoyed melting them in the warm water bit by bit...one letter would come off then they would swish the block round in the water to release the next letter.
As my letters were magnetic, I bought up two flan tins to stick the letters onto.
Once all the letters and the toy were released from the ice block, I helped DD spell her word out while DS managed his on his own.
The finished product...words proudly spelled out with accompanying toy!