LGA - LaGuardia Airport, New York

Of the three New York airports, LaGuardia is generally the most convenient.

There are three city buses connecting the airport terminals to the subway system: Q33, Q48 and Q72. For a newcomer, any of them are fine. There is frequent round-the-clock bus service to the subway. The cost of the bus is $2.00 but the fare box only accepts passes or quarters. That means you MUST HAVE 8 QUARTERS on your person at the time you board the bus. The subway costs about $2 more, but machines there accept credit cards.

The other (wiser) option, if you are going to make more than a single trip, is to purchase a MetroCard pass at the Hudson newsstand at the airport. This will save you the cost of the transfer to the subway.

Here's some more advice on public transit:
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An express bus to Manhattan costs $12, but I always go directly for the subway, since you probably have to take it at the other end anyway.

LaGuardia itself is a ho-hum airport. No free wifi. There are several separate terminals for each major airline. It is unlikely that you can comfortably spend the night here.

VCE - Venice International Airport, Italy

A fantastic alternative to big European airports. On the mainland across the lagoon from Venice itself. You can buy transit passes here covering both the connecting bus to Venice and the Vaparetto boat system within the city (Venice's equivalent to the Metro).

There are two buses to the Piaza De Roma - the entry point to Venice. There is an express bus for about €4 and a local bus for about €2 (or free with the transit pass). Since the local bus takes only slightly longer than the express, and luggage is okay, the express bus seems pointless. The expensive Alaguna boat to Venice also seems totally pointless: Since you need to take a shuttle bus to reach it, you probably would be there already if you had just taken the standard bus.

NEVER buy any transit pass online before you arrive. It's probably a ripoff.

Venice itself is worth a day, but little more. It's kind of like Disneyland: It gets tired fast. There is a Left Luggage office at the airport, so you can leave you luggage there after your flight, tour Venice for the day, then come back for your luggage in the evening.

For cheap lodging, you probably want to stay on the mainland. I found adequate hostel lodging at Camping Jolly, accessible via my transit pass. (See Hostelworld.com)

MAD - Madrid Barajas International Airport, Spain

VERY convenient airport, directly connected to the subway system. Fare is slightly higher than the city fare: about €2 (2008).

CDG - Charles de Gaule Airport, Paris

This appears to be the principle entry airport into Paris, at least for low-cost flights. A shuttle tram connects the 3 terminals. Direct train to Paris Gare du Nord for €8.40. (There is both express and local service, so be sure you get on the express train.) Train takes about 30 minutes, and departs from Terminal 2 and 3. The TGV also stops here.

If I needed a place to secretly camp, I would take the train one stop to the Villepint station, which is in the middle of a big, wild forest park. You can easily and discreetly camp here in darkness only, although the area is exposed during the day. If you stop at Villepint and then continue to Paris, the total fare is nearly the same.

You need to allow plenty of time to depart from CDG. The main bottleneck is passport control, which took me about a 45 minute wait on a low-traffic day. (Check-in for international flights closes one hour before flight time, but even that might not be enough time for you to get to the gate.)

FLL - Fort Lauderdale International Airport

A good alternative to Miami International Airport for getting to South Florida. The two airports are basically interchangible because of a cheap commuter train linking the two. To get to the train from FLL, you must take a shuttle bus to the station, since the distance is too far to walk. The shuttle is free but takes a long time (20+ minutes). The fare FLL to MIA is only about $4.

FLL has free wifi. The areas beyond security close at night. This is not an airport where I would feel comfortable sleeping at night, but I did find a grassy area to sleep nearby (during darkness only).