Best ruin tour is not in Belize - go to Tikal. Get in touch with Tropic if you don't want to sweat the details. They'l book a night in Flores, fly you into Guatemala mid afternoon and you can walk around the town. Get dinner at a lakeside retaurant and enjoy visiting the shops.
Next day you take a bus to Tikal, climb a few pyramids and then have lunch at the jungle lodge. Fly back to San Pedro in the afternoon and out for dinner that night. But, be prepared to pay an extortionist exit fee from Guatemala - if you fly, they gouge you!
Sure, it's more expensive, but a heck of a lot more fun and more impressive than the Lamanai trip.
I've done both and I'd pay again to go to Tikal.