Water - Here is a low profile and obvious way to store upwards of 60 gallons of clean water without really trying. Most homes and apartments have water heaters that are easy to get to.
You have to flush the tank regularly to remove solids and sediment - mark the tank prominently with a big high visibility label stating "purge monthly". This will involve opening the drain valve and overpressure valve to flush out the minerals and sediment precipitated out from heating the water.
If the overpressure valve doesn't work - replace it - water heaters can explode. You need to drain the unit yearly if you have the ability and means to do so (a hose and drain lower than the tank drain valve will work).
If you are getting a new tank - replace the cheap plastic drain valve with a brass boiler valve that won't leak the first time you use it - and then flush it regularly once a month. If you have a whole house sediment or chemical filter even better...you can adjust your flushing schedule accordingly.
When the situation calls for it, turn off the supply valve to the tank turn off the gas valve or electric supply and drain water as needed. You may want to have a butane gas stick lighter on hand to reach the gas pilot if you start it back up.
One half inch of sediment can result in 70% more energy use so do yourself a double favor - save money on heating and keep your back up water clean and handy.
Got a tankless system - see water prep tip 1.