I want to take my family to Mexico for a week in mid-April (yes, in under a month). I’m trying to find a location that would allow us to easily spend half of the day exploring/sightseeing, and the other half sitting on a beach or by a pool. Food is important. We want to see a Mayan ruin or two. We would prefer to stay at a smaller resort/hotel or rent an apartment/condo (budget: up to $400 per night). Since it is our first time in Mexico, I want to go somewhere that is fairly easy to get to from one of the main airports (We will be coming from either EWR or PHL). Thanks.

Can't recommend a specific hotel, but the Yucatan peninsula is realtively safe and people have a better attitude towards tourists.
From my experience, if you like good food, do not go with an all inclusive. It is cheaper to eat out, than to pay per diems. Their luxury level cusine is far from our north american standards. I'm not picky, but I like food to be tasty and well made, whether it is french haute cusine or traditional mexican. Had our best meal at a buffet tucked behind the fast food bar at Chichen Itza, we were they only foreigners. Unassuming mayan cusine prepared by mayan women that barely spoke any spanish.

Some other options: Valladolid (from which you can easily see Chichen Itza, Ek' Balam, and some lovely cenotes) or Merida (from which you can see Uxmal and the Ruta Puuc, particularly if you stay over night along the route, say at in Santa Elena) - or part of your time at each.

thanks for the suggestions.

Check out places in Playa del Carmen. Lots of good restaurants, shops, good beaches...easy to get to ruins from there also.

Mayan Ruins and beaches and upscale lodgings only occur
in cancun.bookit.com Ritz and Meridien 2 great ones from personal experience bidding priceline.com works great for me.
booking.com for no bid good deals still.

Happy Hunting!

I'm more of a budget traveler so my lodging choices - though perfect for us - might not interest you. I'd either fly into Cancún and head to Valladolid for a few nights as kja suggests above, then head to the beach, or head straight to a beachy place like Puerto Morelos, Playa del Carmen, or Tulúm and do day trips to ruins and cenotes from there.

If you click on the Yucatán and Maya Riviera collections below there are links to blogs and detailed travelogues with my recommendations. Happy trails!
http://www.flickr.com/photos/staceyholeman/collections

"Mayan Ruins and beaches and upscale lodgings only occur"

I couldn't disagree more!
There are countless gorgeous resorts on the beach all the way south from Cancun; in addition, several ruins are much more easily accessed (Coba, Tulum, Muyil) from the southern part of the highway.
It sounds as though you want to be at the beach, mostly, so again, Akumal, Playa del Carmen are the best bets for everything you want.