Use a quicklink as a wrench
Quick links are commonly used as rappel hardware, but they can loosen over time and of course work better when the sleeve is locked down tight. If you find a loose quick link on a route, here's how you might be able to tighten it, provided you have another link with you. This lets you apply a lot more torque then you can with just your fingers.
Notes:
This method is meant for field improv. Obviously, the best tool for this job is a crescent wrench or pliers.
It's best practice to install quick links so the sleeve threads down to close, not upwards. That way it's more likely to stay closed.
Quick links are a rather heavy, single use piece of gear, and many people will choose not to carry them. For some routes it makes make sense to have them, for others perhaps not.
It's best practice to use proper CE rated quick links for climbing rather than random ones from the hardware store. CAMP makes inexpensive, super-strong quick CE rated links in three different sizes, read more about those here.
Like many things in climbing, it's a better show than a tell. Here's a quick video I made to show how it works.
