I just booked our spring trip to France, which has inspired me to add the following note to my OP:

It pays to explore fares for multi-city and/or "open-jaw" flights in your planning. In a good many instances, you'll find you can travel to more than one destination for the same price--or even less!--than the cost of a simple round-trip fare.

In our case, we're planning to spend 4 weeks in Paris, but we also had an urge to tack on a few days in the south of France beforehand. As it turned out, and with a bit of jiggling of dates, we were able to book Detroit-Nice-Paris-Detroit for (slightly) less money than a Detroit-Paris round trip.

Google Flights is definitely your friend when it comes to researching such things. For example, when I went on the Delta website to book my desired multi-city trip, it couldn't do it--even though Delta's partner, Air France, offers numerous flights daily between NCE and CDG. But Google Flights helped me figure out all the options. I ended up booking an open-jaw Detroit-Nice, Paris-Detroit trip with Delta and then booked the Nice-Paris leg on the Air France website.

Flights within Europe are often quite inexpensive, owing to a lot of competition from cut-rate carriers. Thus, the total cost of our jaunt to Nice and then Paris ended up costing about $50 less than a simple DTW-CDG round trip. Moreover, I used a wad of accumulated Skymiles (aka "Delta pesos") to upgrade our seats on the trans-Atlantic legs.

C'est si bon!