I do not know we were two over the hill amateurs when we went.

I'd get a guidebook - I like Fodor's and Moon Handbook for Belize. You'll find suggested itineraries and great cultural background info as well as descriptions of regions and specific lodging, food, and transportation advice.

With 7 days I'd split between 2 places - inland and beach. The San Ignacio area and Toledo District in the south are good inland options. Ambergris Caye and Caye Caulker are the most popular island options and Placencia and Hopkins have nice mainland beaches. I've pasted my favorite Belize links below. Happy planning!
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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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Secluded resort on N. Ambergris Caye with snorkeling and flyfishing from shore: http://www.tranquilitybayresort.com
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Very rustic atoll experience, cheap if you have a week: http://www.glovers.com.bz/
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Great jungly place near Punta Gorda: http://www.hickatee.com
Rental apartments in the heart of Placencia: http://www.oneworldplacencia.com
Good Monkey River Trip guide: Jason Williams <wjason628@yahoo.com>
Good Placencia area fishing guide: Wayne Castellanos (dial 011 501 1st from US): 661.1151 or 634.2852
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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)

We did a combo of the San Ignacio area and Ambergris. It gives you a good split between jungle and beach. We went hiking, used PACZ to go over to Tikal to see the ruins, and went canoeing in Barton Creek Cave. The ATM tour is also popular. Ambergris has lots of hotel and restaurant choices, and the snorkeling at Hol Chan is spectacular. We did a split between Chaa Creek and Victoria House, but that may not be what you're looking for. Have heard good things about Mahogany Hall if you want to check that one out. It has more of a B&B feel to it.

Most of our hotels have 20 units or less and have many great options for you to consider on both land and sea. Best trip would be to combine beach and mainland, but if you want to stay put in one location, Placencia is a good choice for both and has a quaint village and a nice selection of restaurants and bars for socializing. I am partial to the Cayo district for all it has to offer, but Toledo is another mainland destination for active adventure.

It all depends on how much appeal inland activities (rainforest, wildlife, Mayan ruins) hold for you vis a vis the water-oriented activities. Some people are keen on both. Some people tilt strongly one way or the other.

Start with the big picture and use gmaps and you tube to learn the area. My must sees are old railroad rights of way which may not be your must sees. Look at the various cayes and the mainland.