Magical swords, magical rings, cloaks of invisibility, seven-league boots -- very popular in fantasy fiction.
Of course, so is complaining about them as plot coupons and MacGuffins. And someone says that they work only through an arbitrary decision of the author. Which notion I walk around, eye a couple of time, poke with a ten-foot pole, and say, "What in any work is not an arbitrary decision of the author?" Of course, if the reader thinks that, it's probably bad, but all that says is that when it's done badly, it's bad. Not useful advice.
Then, there are things that really are bad.
Too many things, for instance. It's not a D&D game. For one thing, there's no danger that they will accidentally sell the one thing they absolutely need; the writer is in control. And unless the world is rife with magical things -- and magical things on par with those that your characters have, so they are not unusual for having them -- loading up with too many magic things means that your characters don't have to use their sense and skills to win.
Though, actually, not having to use their sense and skills is a problem no matter how plausible the things are as possessions, or how few of them there are. If the thing doesn't have a simple, straightforward use, such that the characters will have to figure out how to apply it useful in many situations, it should be perverse. If it's that complicated, it's not going to easy to figure out. A magical jar that can pour out any kind of liquid? Just imagine what could happen if you did it just a little wrong. . . .
And, like that jar, the thing should resonate symbolically with its purpose. Swords, oddly enough, tend to be enchanted to kill things. Rings let you put on and off magical abilities like they were -- rings. Boots for walking. A cloak, which covers you up, for invisibility.
And on top of that, it needs significance of its own. Something you happen to find lying about -- unless it's indicative of the sort of place where you found it -- lacks significance. Give it a history. Make it a gift. Grant it some kind of sentimental value. Give it some details that will make it look less MacGuffiny.