Hi Canukgirl,
I'm not an expert on the Cayo District, but since no one has made suggestions, I'll throw in my 2 cents.
My top choices for a short trip:
ATM - (for me) is a "must see" ; a great mix of adventure, culture & nature.
Caracol - is a wonderful Mayan site.
Mountain Pine Ridge & Rio Frio Cave - Gorgeous Pine Forest for hiking & exploring. Cave area has several pools & waterfalls to swim in.
As far as Bus transport goes from BC here is the National Bus schedule:
http://www.nationaltransportbelize.com/schedule/district/?d=4Have a great trip