We did this one recently (D is 34 months) and it's very interesting what a difference a few months make! This time I froze a few lego men and some 5p pieces in their own domed ice cube (using one of Iz's weaning pots). I only half filled each well first and then put in the items and let it freeze before filling up the rest of the hole. This meant the objects were not all at the bottom (they float) when I flipped the cubes over, but halfway up. Once they were frozen I put them into a large baking tray, added a little bit of water and froze once more.
As soon as D saw the lego men he was hooked. Interestingly he only targeted one ice mound at a time and carefully worked on it until that man or 5p was free. Then onto the next one. I reminded him how the salt would help melt the ice and so he put little piles on the top and then watched as the ice started to crack. I love hearing how his language and understanding is progressing, saying things like "listen mummy, it's cracking like an egg" and "look at that new hole".
It was also a great way to introduce hot and cold. I used pink and blue food colouring to emphasise which bottle was which but D could already easily identify just by feeling. When he'd used all the original water I re-filled the bottles from the icey water in the tray and he quickly realised how much colder the ice made the water.
And for me? I felt like we were freeing Han Solo trapped in the ice - a young Harrison Ford needing my help? I'm down with that ;)