Since your title also mentions Tikal, I'd recommend splitting your trip. Spend some time in/around San Ignacio, cross the border into Guatemala and spend a couple nights close to Tikal.

Then, move to one of the beach destinations. Ambergris Caye and Caye Caulker are closest to the reef, so snorkeling is easiest from those two.

Although there are smaller cayes in southern Belize that are right on the reef. Not much to do there but snorkel, dive or fish.

If those sort of places interest you, look at Southwater Caye or Tobacco Caye, or one of the atolls.

There are also beach destinations further south in Belize. Hopkins is about 10 miles from the reef. Placencia is a 16 mile peninsula in southern Belize. 19 miles from the reef.

Car rental is expensive, we've rented in the past, but, most recently have taken the bus. No car rentals on Ambergris or Caulker, but, you can rent golf carts.

We prefer using the bus system over renting, then fill in with taxis or shuttles if we're off the beaten track. I agree with Jean about splitting your trip. Grab as much time as you can - you won't regret it. I've pasted my favorite Belize/Tikal links below. Happy trails!
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My photo collections with blog, travelogue, and review links: http://www.flickr.com/photos/staceyholeman/collections/

My travel blog starting in 2012 (earlier ones linked with the photo collections):
http://hopefulistinlatinamerica.blogspot.com/

My descriptions and reviews on Trip Advisor:
http://www.tripadvisor.com/members-reviews/hopefulist

Guidebooks I like: Moon Handbook, Fodor’s Guide, and Explorer’s Guide.

Laminated wildlife identification cards: http://rainforestpublications.com/belizeFieldGuides/index.html

A very cool checklist book for Belize for the young or young-at-heart: http://www.amazon.com/Play-Belize-Mesa-Somer/dp/0615198546/ref=sr_1_2?ie=UTF8&qid=1330549187&sr=8-2
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Budget to midrange lodging site: http://www.belizeexplorer.com
Great island and lodging downloads: http://www.belizefirst.com
Informative transportation blog: http://www.belizebus.wordpress.com
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Shuttle recommendation: http://www.parrot-nest.com/belize-shuttle.html
Zoo: http://www.belizezoo.org
Great jungle camp in Cayo: http://www.belizecamp.com
Great budget lodging in Cayo: http://www.thetrekstop.com
Great value hotel with pool in Cayo: http://www.aguadahotel.com
Don't miss cave trip: http://pacztours.net/actun-tunichil-muknal/
Another, less-visited amazing cave option in Cayo: http://www.actunchapatadventure.blogspot.com/
Lodge with lots of adventures: http://www.cavesbranch.com
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Roxy Ortiz - fabulous, fun tour guide: http://www.tikalroxy.blogspot.com
My favorite hotel at Tikal: http://www.tikalinnsunrise.com
Good choice in El Remate, Guatemala: http://www.30minutesfromtikal.com/
Favorite midrange places in Flores: http://www.corpetur.com
Good value budget option in Flores: La Mesa de los Mayas, mesamayas@hotmail.com
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Great inn in coastal Garifuna town: http://www.hopkinsinn.com
Garifuna drumming center: http://www.lebeha.com
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Apartment on the beach on Caye Caulker: http://www.staycayecaulker.com/sailwinds.html
Another Caye Caulker option: http://www.gocayecaulker.com/accommodations/maxhapan-cabanas
Great day snorkeling trip: http://raggamuffintours.com/hol-chan-snorkel/
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Very rustic atoll experience, cheap if you have a week: http://www.glovers.com.bz/
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A few more for good measure (2012 trip):

Great jungly place near Punta Gorda: http://www.hickatee.com
Off the beaten track diving and snorkeling, Sapodilla Cayes: http://www.reefci.com
(haven’t stayed there yet thanks to Hurricane Ernesto in August 2012)
Rental apartments in the heart of Placencia: http://www.oneworldplacencia.com
Secluded resort on N. Ambergris Caye with snorkeling from shore: http://www.tranquilitybayresort.com

Thank you so much for your suggestions. we will check things out. I/we appreciate the time it took.

We stayed on Ambergris and snorkeled at Hol Chan - it's some of the best we've ever done with sea turtles, big rays, and lots of colorful tropical fish.

sounds great, thanks