What makes a great day trip? Going somewhere different -- and not taking too long to get there. Your destination has to have enough fascination to fill a day, but not so much that you feel you're leaving something out.

In Europe, high-speed trains and ancient historical legacies make for a wide range of possible day trips, on comfortable, speedy rails. But that sort of expedition isn't limited to Europe. Here in the US, Amtrak and local rail systems have a few tricks up their sleeves to get you to entertaining destinations for a day.

I've picked five day trips for five popular cities than you might not have thought of before. All are accessible by easy, frequent trains.

Trip #1: London to Lille and Tournai

Paris is not a day trip. No way, no how. Yes, with the high-speed, luxury Eurostar train you can easily get between London and Paris in a few hours, but trying to pack Paris into a day will leave you frustrated, angry, and with a sense of miserable incompleteness. That's not a good day trip.

There's a better one-day alternative on the same high-speed line: Lille/Tournai. Lille, only an hour and 50 minutes from London ($80 and up each way) gives you the atmosphere of a classic French city that flourished in the 1700s, in a convenient, compact form. Try to go on a festival weekend. On August 31, for instance, Lille will be throwing what is possibly the world's largest flea market, the Braderie whereas on June 11 this year, the city will be full of food, music and a parade of papier-mache giants as the city celebrates the anniversary of its founding.

Then hop a local train (20 minutes, $9.38 each way) just across the Belgian border to Tournai, a medieval city with a glorious cathedral and some magnificent works of architecture.

The 8:39am Eurostar will get you to Lille by 11:26am and trains to Tournai leave at least hourly. You'll even have time to catch a real French dinner before making the 9:06pm train back to London, arriving 9:56pm. That's one day, in three countries.

Trip #2: Paris to Reims

Yes, you could get to London from Paris in a day, but that wouldn't do justice to one of the world's most dynamic cities. Instead, try something different: hop on Europe's newest, shiniest high-speed train line, the TGV Est, for the 45 minute, $43 journey to Reims. The TGV Est, which debuts on June 10, is also Europe's fastest train -- they ran a demonstration train at over 350 miles per hour, though the passenger service runs at a more demure 200 mph or so.

Aside from the spectacular cathedral, the fun in Reims is touring the champagne houses and tasting the famous wines. You can wander on foot through a labrynth of underground tunnels (at Mumm), zip along in a little electric car (at Piper-Heidsieck), or ogle the Gothic buildings of Pommery. Understandably, Reims also has several excellent, but quite high-end restaurants making good use of the local wines. Try lunching at one before grabbing the 6:19pm train back to Paris for dinner.

Trip #3: New York to Philadelphia

Travelers to New York, especially from Europe, try to cram in Boston and Washington as day trips. But they're a little too far, and, especially with Washington, a little too complex for a one-day journey. Philadelphia, on the other hand, is a perfect fit.

Of course there are several days' worth of things to do in Philly. But the peaceful vibe of the old city, with its gracious Federal townhouses and low-stress history lessons, are a perfect counterpoint to the nonstop, screaming intensity of New York. Our Philadelphia walking tour lets you take in the sights and stop for a cheese steak at any of the many local joints we recommend. If you can stay for dinner, though, I suggest you do. Philadelphia has some startlingly good fine dining restaurants, and the prices can be half what you'd pay at similar places in New York City. That's how my wife and I ended up at Morimoto on our first anniversary -- and got away with dinner for two for under $100.

Amtrak provides the smoothest, swiftest service between the two cities, with the trip taking only an hour and 20 minutes -- though the fare ($45 and up each way) can be hard for some to swallow. There's a slower, cheaper option: take the local New Jersey Transit train to Trenton and switch there for a local SEPTA train to Philadelphia. That route costs only $17, but takes two and a half hours each way.

Trip #4: Boston to Providence on a Saturday night

Providence, Rhode Island is a city that doesn't really trumpet itself. Yet it's quietly become a friendly, attractive town, with a signature art event that's worth a Saturday day trip from Boston: WaterFire ( WaterFire sounds pretty lame in print: it's a bunch of fires lit on the rivers downtown at night. Yet between the darkness after sundown, the light and smell of the bonfires, and the ambient music played on the riverside walkways, it all comes together into a mellow and unusually relaxing evening. Get to Providence for lunch and spend a few hours strolling the College Hill historic district, shopping on Thayer Street and comparing Brown students to their more amped-up Cambridge cousins.

You can't beat the price of the MBTA commuter train between Boston's South Station and Providence. Take the 10:05am train down to Providence and the 10pm train back on a Saturday (travel time one hour, five minutes) and you'll only be out $15.50 for the round trip.

Trip #5: Barcelona to Tarragona

Barcelona is one of my favorite cities, and I could easily rattle off a half-dozen terrific day trips out into Catalonia or the local hinterlands. But for this story, the best city to twin with Barcelona is Tarragona -- a compact museum of Roman ruins and Catalonian history. (Girona is also a good bet, but it's a more popular choice; we're leading you a bit off the beaten path.)

Tarragona has a well-preserved Roman circus, ampitheater and medieval walls, along with at least one extremely good, reasonably priced restaurant, Epicuri. That makes the city ideal for lunch and a stroll. The most affordable way to get from Barcelona to Tarragona is the one hour, 20 minute ride on the "Catalunya Expres" train, for $7.50 each way. Taking the 8:47am train gets you to Tarragona at 10:07am; the 6:57pm return trip gets you back by 8:27pm, time for dinner in Barcelona. (Later trains leave Tarragona as late as 11:06pm.)

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