Without a doubt, go to Mexico. We went Easter 2006, when our girls were 7 and 9 and 18months on they are still talking about it. Infact the eldest took photos into school today of the Mayan Pyramids for her History class. There is so much to see and do you needn't be confined to the hotel, infact to not venture out would be criminal.
There are some great hotels in mexico, but we stayed at the Iberostar Tucan in Playacar- they have a kids club that runs all day, infact they don't even need to return to you to go and eat unless they want to. In the fortnight we were there the girls were in productions of both Grease and Cats- with full costume and make up. Not once though did we feel overun by children and it was the Easter holidays- lots of American children for them to make friends with though.
Great for adults too- couldn't fault the service and food. The A La Cartes dealt with the monotony that some people don't like with AI and cocktails were on tap all day. We even managed a sit in the adults only swim up bar with a Mojito- bliss!!
It is our best ever holiday to date, so one well worth considering.
Pam