Here's a riddle: When is a free night not a great deal? To find out,  follow recession travel rule no. 28 and compare apples to apples.

Wyndham's "5^th Night Free Plus $100" deal seems straightforward at first. When you stay at least five nights at any in a long list of Wyndham properties in Mexico and the Caribbean, you'll get your fifth night free plus an American Express gift card worth $100. Getting one night free during a five-night stay translates as a 20% reduction of your bill. That seems like a nice savings, though in this economy many hotels are offering a fourth night free (translation: 25% discount) or a third night free (translation: 33% discount).

We decided to price-check this promo for the Wyndham Sugar Bay in St. Thomas, a family favorite that's great for snorkeling and iguana encounters. Best of all, it offers a free kids' club for children 4 to 12. For some families, that perk could save a few hundred dollars during a five-night stay.

Think getting a free night is a no-brainer? Not so fast. The "free night" rate may not be calculated at the hotel's lowest available rate. At the Wyndham Sugar Bay, for instance, the "5th Night Free Plus $100" package rate starts at $219 a night, compared to the advanced purchase (and non-refundable) rate of $170 a night. So, after you subtract the cost of the free night, you'd pay $876 for a five-night stay versus $850 with the advanced purchase rate. Suddenly that "5th Night Free" deal isn't looking so hot. (True, when you factor in the $100 gift card, the package nets out as a $74 savings. But you wouldn't be able to use the $100 gift card on your trip. You'll receive your gift card in the mail six to eight weeks after you get back home. )

But wait, hold the phone! When we checked Wyndham Sugar Bay's special offers page, we spied the "Caribbean Vacation Stimulus Package," which gives you two free nights when you buy two nights, plus a $100 food and beverage credit. The rate for this package starts at $219 a night if you travel between August 25 and October 31. (It jumps to $294 a night in November.) On a four-night stay, that works out to a 50% savings plus a daily credit of $25. The upshot: For fall getaway in September or October, you'll spend just $110 a night, plus you'll have an extra $25 daily credit to use in the restaurants. That means your daily spend will be a whopping $85 a night lower than the lowest available rate. Jackpot!

The "Caribbean Vacation Stimulus Package" is good through November 30 and can be booked online with promo code PKGCARIB1. The "5^th Night Free Plus $100" deal is good through December 30. To reserve either deal by phone, call 800/WYNDHAM.

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Recession travel rule no. 27: Visit Travel Ticker