Plumbing Repair Kit to Carry With you. Just in Case...

All of the above is good advice, and the single most useful PEX fitting I've found to have on hand is an inline shutoff valve. The valve can be used as a valve of course, to control the flush water to a toilet for example, if the existing flush valve won't shut off. But it can also be used as a straight coupling or an end cap, making it useful in a number of situations. I prefer the no tools Watts, Sea Tech, and Sharkbite style fittings that only require a sharp knife for installation.