Whereabouts are you going on Riviera Maya? If you're going to Playacar or Playa del Carmen then Xcaret is only a 5 minute taxi ride away so I would do that solo without using a tour company. Cancun is only an hour away by air conditioned coach which only costs about $6 return from Playa del Carmen bus station and runs every 10 minutes. Snorkelling trips can be booked from a booth at the start of 5th Avenue (oppsite MacDonalds) for a reasonable price and you can sometimes haggle them down in price. For a lot of the trips the tour operators actually only provide the transport, the attraction provides everything else e.g. eco parks, zip lines, cenotes, ruins etc. For long trips like Chichen Itza the transport may be better with Thomsons (air conditioned coach), but check with the local operators when you get there.

Cheers

Keith