Obsidian and stone cauldrons.

Add the fluids mixing mechanics to cauldrons but keep the pickaxe level requirement.

If water flows over a lava cauldron, the lava in the cauldron would turn into obsidian. the player can still remove the cauldron with the block, but he couldn't just remove the obsidian block without the correct pickaxe (maybe a right click with the right pickaxe would be enough).

In the case of stone, it would be the same mechanics. But instead of a cobblestone, the player would receive a stone. Giving a good reason to have a cauldron at the player's base that would not replace furnaces / silk touch.