Since you're interested in the possibility of stopping off somewhere interesting between Paris and Venice, rather than taking the sleeper directly (as mentioned above), consider Turin. I've stopped off there to and from Italy a few times in recent years, not having done so before, and found it interesting. (And it's probably more "manageable" than the rather larger Milan, if you don't know either place to start with, and don't have a bit of Italian.) There are the direct Paris-Turin trains - via quite a scenic route, and the Turin-Venice connections are fast and frequent.
If you don't mind a change between Paris and the next destination, and between there and Venice, you might consider the Ligurian coast (the part running east from the French-Italian border). A train from Paris to the French Rivera, followed by a train along the coast into Italy, is in parts a very attractive journey - though it would take most of the day. Liguria (most parts at least) to Venice can be done with one change in Milan. If you're into cities, then Genoa is the main one in Liguria; but there are loads of nice smaller places along both the French and Italian coastlines there, where you can spend a pleasant day or two.
An alternative interesting option, between Paris and Venice, is southern Switzerland. There are direct trains Paris-Lausanne (and others with a change in Geneva); you can either stay in Lausanne (which is on Lake Geneva), or go a bit further east by hopping on a Swiss local train that goes towards Montreux and along the valley via Sion to Brig. It's an attractive area (and much of it is in areas of Switzerland where they speak French rather than German, if that's a factor in terms of language knowledge). From Lausanne (or indeed the main towns onwards from there in the Brig direction) there are some direct trains to take you on to Venice, and others where you change in Milan.
If you find booking a hotel in Venice rather pricey (and I hope you'll show solidarity with the remaining beleaguered residents of Venice by not using something like Airbnb, which is a factor in the depopulation of the city, turning it into a Disneyland rather than a real living city), then there are places a short train hop from Venice (en route from Milan or wherever), which are cheaper to stay in, and enable you to easily spend all day in Venice - I've done it staying at a hotel within sight of the station in Padua, which is just 20-odd minutes down the line. And Italian train prices - for a Brit - seem absurdly cheap!
Between Paris and Barcelona, there are some obvious places you could stop en route, like Avignon or Montpelier - I've spent pleasant times in both of them in years gone by. Friends recommend Nimes too, though I've never stopped there myself. NB, however, that some of those places in southern France don't make as convenient a stopover as they used to - they've built several "bypasses" so the TGVs don't have to slow down getting in and out of all the cities they call at, with the result that some trains stop at new out-of-town stations, quite a hike from where you want to be. Which means that in some cases, getting from Paris to X, or from X to Barcelona, requires either taking a slower train, or changing trains en route, if you want to avoid being dumped in the middle of nowhere. Yes - it's a bit silly, and plenty of the locals who live in those towns don't like it either.
Some of the places you might go to - as you probably know - require a change of stations, like London does. Paris, for example, and Milan too sometimes. But if they're places you're spending a day or two in anyway, rather than just changing trains en route, then of course that's not really a hassle.