put the grenade in a block of ice, without the pin (nevermind how)

lol interesting, I was thinking the opposite, how you would do jello and how boiling water might interact with the pin and other mechanisms. I would place the grenade in a bowl, and then from a very safe distance, something like a kettle with a rope attached, pour the boiling jello. But I wasn’t thinking of having the pin removed. Since this is like, supposed to be a prank. But even then, who knows after all that heat and water contact with the mechanisms.

The ice case seems simpler to me, at least for grenades where you hold the handle with pressure pushing towards the grenade. You could pre-freeze a molded form sufficiently sized to fit the grenade with the handle pressed. But not allow it the space to release until the ice is melted. Then once the grenade has been inserted, you could fully freeze more water around it to make a solid block, presumably for aesthetic purposes. Still you might worry about metal expanding and contracting under different temperatures, when you do the second freezing process.

Iraqi insurgents would do this with soviet mortars, freeze a mortar to the side of the tube, then it would take 45 minutes or so to thaw and thoom ........... BOOM