I placed some water in a random spot earlier and it turned into a block of ice. Now I can't figure how to do it again...
did, dod, dud, etc...
You placed it in a Tundra/Taiga biome. To do it again, you have to place water in a Tundra/Taiga biome again. Although you can't pick it up.
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oh, I see. Ok...
did, dod, dud, etc...
Look for ANY section in your world where there is snow (that's an indication of a "Tundra/Taiga" biome), or you have frozen water. Make a nice "mold" of Dirt Blocks (or any blocks, really), and pour Water into a 1x1x1 hole. It'll freeze, eventually.
If you want to take it to another location, use Pistons (Sticky, or otherwise). Ice Blocks will NOT melt due to being in another biome. However, most light sources will melt the Ice back into Water (you can use Redstone Torches, safely... but Lava and regular Torches will quickly melt the Ice).
You can also make an "Ice Block Generator", using a 3x1x1 Water trough. Keep the Water sources on the left and right "covered", but leave the middle alone. It will freeze... allowing you to use a Piston to push the Ice Block up with a standard Piston, and will retract, allowing the water to reform the "infinite water block"... you can then use more Pistons to shove the newly formed Ice Blocks to another section of the map (you can generate up to twelve blocks of Ice in a row, before the thirteenth causes the piston to stop being able to shove Ice Blocks).
Just keep your light sources ~low~, and you'll be fine.
You could actually generate a theoretically infinite amount of ice blocks that way, by using another piston to push the last block upward and, optionally, a third piston to push it back in the same direction, and repeating that pattern.
That said, the helpfulness of pistons automatically moving generated ice blocks is minimal at best. If you wanted to construct something like an igloo, you'd be far better off using a single piston to push each individual ice block to where you need it (picking the piston back up after each block of movement). It'd be tedious, but you could do it. Any large building of ice I would just totally skip until we get enchantment (and even then I'd skip it since it'd be ridiculously easy to accidentally ruin).
Oh boy! I can't wait to bomb some dodongos!
Oh, indeed, you could make an "infinite" amount of Ice Blocks like that... or, you can even go so far as to just rebuild the generator itself, in another location.
...though, really, you bring up a good point... and in a round-about way, you also provide a good solution. Wait for enchantments, and use "Silk Touch" to use on the Ice Blocks being ~produced~ by your Ice Block Generator... being careful to not break the device itself.
...though, given that Creative Mode is going to be a "Toggle" system, you don't even need to do that... just load an Ice Block into your inventory hotbar, and have at it... kind of kills the need for ~any~ generator.