Internal travel is a big part of the Belize experience, and can also be a significant cost.
Consider which mainland adventures you would like to enjoy, and perhaps put them at the start of finish of your trip?
What time do you land and then eventually leave Belize?
What time zone are you arriving from?
Can you do a trip the day you arrive and then stay in Corozal for instance, and take the Thunderbolt across the next morning?
How many in your party...if its 4 or more, perhaps an operator will pick you up at Goldson and drop you at a hotel later?
Hotels will be 50-60% the price of a similar San Pedro lodging, and you'll save on flights
The more info you give, the better the answers are likely to be