For the trip to Lamanai, I would recommend booking thru Tropic Air Tours. They will provide a small airplane and you will depart at 7:30am - arriving at Orange Walk at about 8am. By taking this flight in the morning, you will arrive at Lamanai a about 9:30 (you will have about a 1.5 hour boat trip on the New River,where the guides will point out the flora and fauna of the river), but most importantly, you will arrive BEFORE the cruise ship tourists and your tour will be nearly complete when they arrive. We were given a nice lunch before returning. Your pilots will probably accompany you throughout your trip and have you back to AC by 2pm. BTW this way you will miss out on several hours on a boat across the bay to Corazol Town and then up the river to Orange Walk. Definitely worth the extra cost (about $35 extra per person).
But I am sure that you will enjoy your tour either way.