13-Nights Viva Mexico: Ancient Ruins & the Mayan Riviera - 18-to-Thirtysomethings from $1,571

About 25 mi/40 km northwest of Chetumal is the town of Bacalar, home to Bacalar Lagoon (the second-largest lagoon in Mexico) and the Spanish-built Fort San Felipe (noted for its museum of weapons and military uniforms but not always open).

Bacalar is in the southeastern part of the Yucatan peninsula, a four-hour drive from the Cancun airport and a half-hour drive from the Chetumal airport.

Bacalar was named one of Mexico's Pueblos Magicos (magical towns) by the Secretary of Tourism. It is famed for its spectacular Laguna de Siete Colores (Lagoon of Seven Colors) and the historic Fort of San Felipe, dating from 1733, along with its Museum of Piracy. The lagoon, which is named for the many shades of blue in its crystalline waters, offers an array of watersports, including swimming, kayaking, waterskiing and pontoon boating.

Take time to visit the archaeological site of Dzibanche. During the classical Maya period, from AD 200 to 800, Dzibanche was a regional center so vast that it had 22 separate plazas; so far, only three have been excavated. Archaeologists postulate that priests walked some 1,800 mi/2,900 km from Teotihuacan, near Mexico City, to perform ceremonies at Building Six, a Teotihuacan-style stone base topped by a pyramid.

Dzibanche's Temple of the Cormorants has hollowed-out burial caves where lower-ranking nobles were interred. And inside the Temple of the Owl, archaeologists found a royal palace with gold from Oaxaca. Trading throughout the Maya area was extensive, and the temple itself is in the Peten architectural style, which is found as far away as Tikal in Guatemala.

About 40 mi/65 km west of Bacalar are the Maya ruins of Kohunlich, which include a temple with giant stucco masks. This city was occupied from the sixth to the 12th or 13th century and was subsequently abandoned. The Temple of the Masks was built about 1,500 years ago.

Also nearby are two other impressive Maya ruins, Becan and Chicanna. Becan flourished from AD 600 to 900, and about 50,000-60,000 people lived there. The site is famous for a unique moat that surrounds it and a covered stone passageway with false arches.

The only major Mayan city built next to the sea, Tulum, Mexico, enjoys a spectacular setting, perched on a bluff above the Caribbean.

The city was still inhabited when the first Spaniards arrived, and it was said to be larger than Seville at that time. What remains today, however, is a rather small collection of buildings, at least when compared with such Mayan sites as Chichen Itza.

Unfortunately, Tulum has become a victim of its own success, and most of its buildings have been roped off to protect them from being damaged by large numbers of visitors. The best time to visit is early in the morning before the cruise-ship and tour-bus groups arrive—this way, you'll also avoid the hottest part of the day. Or take a snorkeling trip from a local tour company and see the ruins from the water instead.

In addition to the beach and ruins, downtown Tulum is a pleasant spot for a stroll. It has a little plaza with a church and a street full of restaurants, bars and shops. At night, some of the bars offer live music and a local ambience.

On the outskirts of Palenque, located near the Mexico-Guatemala border approximately 300 mi/485 km southwest of Merida, lie some of the most impressive Mayan ruins in the country. Climb the tall Temple of the Inscriptions and then descend to the site of the impressive, restored crypt of the Mayan king Pakal. The crypt itself is closed off to visitors, but there's a replica at the Museum of Anthropology in Mexico City. Investigate other ruins to look at carvings and bas-reliefs.

We consider Palenque well worth a visit, but it is extremely hot and humid in the summer. To avoid the worst of it, get an early-morning start.

If you leave the park following the Otulum River down the hill, you will discover several excavated temples and get to many pretty waterfalls cascading into natural bathing pools, a great place to cool off after wandering around the ruins. Some pleasant hotels are found in the village, if you wish to overnight in the area.

If you have time, travel 60 mi/100 km south to the Agua Azul waterfalls. These turquoise cascades are some of Mexico's natural wonders (just try to ignore the tacky food huts dotting the lower end of the falls). You can swim there, but do so with care—several people have drowned.

From the Alameda, a leafy center of activity since Aztec times, to the Zona Rosa, a chic shopping neighborhood, Mexico City offers endless options to urban adventurers.

Founded by the Aztecs as Tenochtitlán in 1325, Mexico City is both the oldest and the highest (7,349 ft) metropolis on the North American continent. And with nearly 24 million inhabitants, it is the most populous city in the world. It is Mexico's cultural, political, and financial core -- braving the 21st century but clinging to its deeply entrenched Aztec heritage.

You only need to stand in the center of the Plaza of Three Cultures to visually comprehend the undisputed significance of this city. Here, the remains of an Aztec pyramid, a colonial church, and a towering modern office building face one another, a testament to the city's prominence in ancient and contemporary history. Located at the heart of the Americas, Mexico City has been a center of life and commerce for more than 2,000 years. The Teotihuacán, Toltec, Aztec, and European conquistadors all contributed to the city's fascinating evolution, art, and heritage. Although residents refer to their city as simply México (meh-hee-koh), its multitude of ancient ruins, colonial masterpieces, and modern architecture has prompted others to call it "The City of Palaces."

The central downtown area resembles a European city, dominated by ornate buildings and broad boulevards, and interspersed with public art, parks, and gardens. This sprawling city is thoroughly modern and, in places, unsightly and chaotic, but it never strays far from its historical roots. In the center are the partially excavated ruins of the main Aztec temple; pyramids rise just beyond the city.