Services - work that requires human knowledge and physical effort. Including the design and construction of new things.
Also: Things that can't be replicated. And things that could be replicated, but are known to have never been.
George O. Smith (no, not "Doc" Smith. This is another golden-age sf writer named Smith) explored some of this question in the later stories of the "Venus Equilateral" series. The gang had invented a replicator, pretty much as good as Trek's. And without a non-copyable form of currency, the economy suddenly collapsed. Suddenly everyone could have all the material things they could want, for pennies on the dollar prices. Including dollars. And twenty-dollar gold pieces. And high-value rare watches. And...
But all of those things had to be copies of things already existing. As one of his characters pointed out, suppose a new type of tech was developed, and you wanted a new gadget that exploited it? Well, someone has to design and test a prototype. Any of the engineers in the VE stories could build a prototype - and it would work very well - but it wouldn't necessarily be something you'd be proud to show to your friends. So someone else has to design an attractive and functional and durable enclosure. Someone else has to build the prototype of that. Etc., etc., etc.
Now how will that effort be paid for, when the value of so many material things has fallen through the floor?
Conversely, who's going to remove your gall bladder, should that become necessary? And how will you pay them... when anything you have, they could replicate for themselves? Or buy from a "repli-factory" for pennies? Presuming anyone wanted pennies.
Answer: You'll have to trade some service you can do, for the service they can do. A gall bladder removal's worth of washed cars and polished floors...
Or something far more mundane: If you want someone to clean your house or wash your car, how will you pay them? I suppose you could just replicate a new (clean) car or a new house, but we have to assume the tech doesn't scale forever; you can't just plop a new house where your current one is and you can't replicate spaces for houses to be. And there should be an energy cost, at least.
As the writers of the "Ferengi" did, the gang of heroes in VE came up with the spiritual ancestor of gold-pressed latinum, which could not be replicated (it exploded violently when under the "scanning beam"). So at least now they could make currency with it, a medium of exchange.
But there's still the problem of what work you can do to earn some of it so you have it to spend. It has to be work that can't be replaced by cheap replication. That rules out most work that involves manufacturing things. Yes, there is the work of creating new things... but there's not that much of that to be done. (Sorry, but George O. Smith never addressed this problem fully.)
One other nice idea in the stories was the concept of "certified uniques" - things that are known to never have been replicated. There might be more than one of something, as today there are, for example, more than one of some rare comic book or baseball card in existence. In the post-scarcity society, since things that have been replicated are damn everywhere, there is no longer any status in owning, say, a Tag Heuer Monte Carlo, or an Apollo-era Omega Speedmaster, or an original Babe Ruth baseball card.
But in contrast, "certified uniques" became even more valued than they were post-replicator. And since replicating them would destroy their value (both monetary and sentimental), the owners were motivated to not let that happen.