we just came back from Riviera Maya and want to share some experience: if you stay at Riviera Maya and want to see the Tulum Ruins, I suggest you take a taxi instead of join the local tour, as it's more flexible, time-saving and of course more cheaper. It costs us (3 persons) $100 USD and only takes 3 hours to visit Tulum which is one of most popular Mayan ruin sites in Mexico. We asked a taxi service via the bellboy of hotel and the taxi driver provided us a round-trip
for 1200 Peso, and we ended up paying in USD plus a tip. Secondly, DO NOT exchange the peso with USD at airport, because the rate is terribly bad, we exchanged $50 USD at airport at the ratio of 1: 14, but at resort/hotel or local market, they used the ratio of 1:18 or 1:16 to convert when you directly use USD..