Pre- River Cruise destination recommendation???

It all depends on where else you've already been and what your interests are. Paris is certainly worth a second, third, fourth etc. visit. London's not a bad trip to Amsterdam, via train through Brussels. If you want to fly in from somewhere, you've got even more options. But you've asked specifically about Paris - worth returning? Yes.

Maybe fly into Brussels and spend a couple of nights in Bruges, think the train from there through Brussels would be fairly efficient. Bruges is around three hours from Amsterdam by train. You might also consider Amsterdam itself or other cities in the Netherlands. (I have never been to the Netherlands, but I am sure there is much to recommend it).

I don't feel that Bruges has enough for three days, but you could stay in busier Antwerp (which may even have a daily nonstop train or two to Amsterdam) and easily train to Bruges for one day. Likewise, you could sleep in Amsterdam and take a one hour or less commuter train to the interesting cities of Haarlem, Alkmaar, Leiden, Amersfoort, Utrecht, and a little farther, Delft and or Den Haag. If the boat's limited tour options don't include Keukenhof gardens, that will be open by then, and is worth a full day - booked excursions will be available if you don't care for public transportation.

Another vote for Belgium. You could base in Gent and visit Brussels, Antwerpen and Brugge before moving on to Amsterdam.

Do you mind telling me what cruise line you are taking??
My Rhine cruise originated in Amsterdam and Day 1 and 2 docked in Amsterdam was deceiving. Day 1 was boarding the ship with "on your own" time in the late afternoon and evening. Day 2 was an escorted bus tour that included the VanGogh museum and in the evening there was a canal cruise.
There is a lot to do in Amsterdam so I was thankful we had arrived early and had three days to explore and take a day trip in the country because the "two" days on the cruise itinerary was insufficient to see this amazing and wonderful city.
So my advice is to look closely at your cruise itinerary and see if you really will see enough of Amsterdam. .

Fly in to Brussels and stay in Ghent, with 3 days for ...
1. Brugges
2. Brussels
3. Antwerp
An overnight in The Hague en route to Amsterdam would also be an option.

I'd agree with Elaine that Amsterdam is worth more than the time your cruise line will give to it. We spent five days there and could have spent more. A side trip to Haarlem would also be good for another day. My advice would be to fly into Amsterdam, spend those three days on your own there, including Haarlem, then board the ship and do their tour or whatever it is. Try to find out what sights the cruise line will be covering so you don't duplicate.

As for Paris, of course it's worth multiple visits, but if you've already done "all the 'tourist' stuff" you may have had enough. But I've done pretty much all the most popular sights there and I'd still go back in a heartbeat.

You could spend the time in London, then take a short flight from City Airport to Schiphol.

I also agree with Elaine.. don't take staying in Amsterdam off the list. If your cruise doesn't include any of the museums... you must see both the VanGogh and the Rijksmuseum. The Rembrandt house was memorable. We also did the Heineken Experience, as well as tasting the great food specialties. We wandered around and loved every minute. We wanted to go to the flower auction, but had no time. We were in Amsterdam 2 full days before our river cruise started. If we had 2 more we would have done a day trip out to the countryside. We never saw the red light district.

There are lots of close places. Bruges, Brussels, Ghent, The Hague, Haarlem, Utrecht. Take a look at what might interest you.

My personal opinion is that Amsterdam has interesting canals and architecture but I like other places in the area better because A'dam draws crowds that are specifically attracted to the pot and red light industries.