Scott mccartney

The strong dollar means that 2015 is the best summer in seven years to go to Europe. WSJ's Scott McCartney discusses. Photo: Getty

March 18, 2015 1:14 p.m. ET

The Seine is on sale.

The strength of the dollar against the euro has created the best buying opportunity of the decade for Americans going to Europe. Even famously expensive cities like Paris have seen prices plunge. Hotel rates have tumbled for travelers with dollars to spend, and new competition this summer on trans-Atlantic routes has brought down airfares a bit, too.

Travel agency Orbitz says prices for Paris hotels booked for stays between June 1 and Aug. 31 are down 10% so far this year, to $231 from $256, based on reservations made up to March 10, compared with the same period a year earlier. The average airfare to Paris from the U.S. is down 14%, to $1,333 from $1,541.

London, Paris, Dublin, Rome and Barcelona are the most popular European destinations for Americans so far this year in Orbitz bookings. Overall hotel rates reserved are down 3% and prices of airline tickets booked down 6%.

“It’s going to be a pretty busy summer on the North Atlantic,” says Craig Jenks, who tracks airline capacity between the U.S. and Europe as president of consulting firm Airline/Aircraft Projects.

Now might be a good time for American travelers to prepay for European hotels to lock in current low prices.

The strength of the U.S. dollar and weakness of the euro, the currency of 19 European nations, sent the euro to a 12-year low this week compared with the U.S. dollar. One euro currently costs about $1.09, down 22% from $1.39 a year ago. Four years ago one euro cost almost $1.50. The British pound cost $1.72 last July but now is worth $1.50.

Seattle retirees Randy Loomans and Anne Wetmore had planned to tour Asia later this year. But the cheap euro led them to book a monthlong tour in Europe for September.

“We’ve just been searching our life away for a trip to Italy,” Ms. Loomans says. They figured a month in Europe would cost them about $10,000 each. But now they have booked a trip to Rome, then a week in a villa in Tuscany, then stays in Florence, Venice, Paris and London for a total of $4,400 each.

The villas division of cruise and leisure-travel company World Travel Holdings says business in Europe is up significantly over last year for Americans, and not just because more people are booking trips.

“We are seeing a clear increase in the length of vacations Americans are taking, more multiple-week bookings compared to last year,” says Steve Lassman, vice president and general manager of Villas of Distinction.

In addition to longer stays, customers are loading up on cooking classes and other add-ons because prices are so cheap, Mr. Lassman says.

Hotels in some European capitals for summer do offer some eye-popping rates. For a week-long stay June 4-11, the Ritz-Carlton in Dallas is more expensive than the Ritz-Carlton in Vienna—$463 per night compared with $401, according to a recent search of Hotels.com. Le Meridien in Barcelona is cheaper than Le Meridien in Santa Monica, Calif.

The Westin Palace in Madrid is half the price of the Westin Copley Place in Boston for the same dates. And you can pay $385 a night at the W Paris Opera for those June dates, or $445 at the W New York Union Square.

STR Global, which tracks hotel data for the lodging industry, says the average room rate in Paris was up 4.7% in January, to 230 euros. But in dollars, it was down 13% to $260. Average rates in Rome in January were down 2.8% in euros, and a whopping 19.2% if you’re paying in dollars.

For airlines, the weakness in the euro and strength of the dollar is a mixed blessing. While Europe gets cheaper for U.S. travelers, the U.S. gets a lot more expensive for Europeans to visit, meaning demand for seats from the European side of the Atlantic should weaken.

Major carriers say that airfares have come down slightly for trans-Atlantic trips. That’s due to the economic weakness in Europe, as well as the addition of more flights, both by incumbent airlines and European discount carriers pioneering new strategies.

Indeed, the average fare for travel between May 1 and Aug. 31 from the U.S. to Europe and back is down 2.4% to $1,415 so far in 2015 compared with last year, according to Airlines Reporting Corp., which helps process tickets booked by travel agencies. For trips sold in Europe for travel to the U.S. this summer, prices have fallen 12%, to an average $879 round-trip, ARC says.

Those cheaper prices are enticing Europeans to keep traveling. A spokesman for Lufthansa says it expects no drop in European passengers headed to the U.S.

Air Berlin is increasing the number of trips it flies each week between Düsseldorf, Germany and both Los Angeles and New York, and between Berlin and Chicago.

The airline has been restructuring to boost business in the U.S. and is actually making its lowest fares more available in North America than in Europe “to get more dollars into the company,” spokesman Aage Dunhaupt says. Many of the airline’s expenses, including fuel, are paid in U.S. dollars.

Smaller U.S. cities are getting new service from the European discounters, too. Condor Airlines of Germany will fly twice a week to Providence, R.I., from Frankfurt beginning June 18. The summer service will mark the first time Providence has had a direct flight to Europe.

Such flights are aimed at European bargain hunters, but there’s no reason Americans can’t take advantage of them, Mr. Jenks notes. Condor’s lowest fare right now for July 16-23 was $876 round-trip on Monday. The cheapest nonstop round-trip ticket from Boston to Frankfurt on the same dates was $1,635 on Lufthansa.