wow, that is small...
If you're not planning on using it any time soon, just keeping water in it is probably not going to help prepare it for fish. (since there is no source of amonia to feed the nitrifying bacteria, the tank will not really 'cycle')
Another approach is the just keep the filter for this one in your main tank. Similarly, you may be able to just put some of the filter media from this filter into the filter for your main tank. This way, when you need the spare tank for an emergency, you just move the smaller filter into your spare tank(or just move the media back to your small filter if you tried the 2nd idea), fill with dechlorinated water, and your tank is instantly ready.
Oh, and yes, that us a cute little tank. Hopefully you won't need to make use of it as a hospital, but good to have just in case!