Fiona,
The Caribbean Beach is a large, sprawling resort. So picking a good building can minimize walks significantly. Make your requests when you do the online check-in, or call the resort (407-934-3400) directly within 10 days of arrival.
Assuming that you want to be near the resort amenities, what you want to do is stay near the Old Porte Royale Centertown. This is
NOT the building where you check in, which is the Custom House. Often people will ask to be near the check-in area, and mistakenly end up pretty far from that main amenities hall.
The best buildings, if staying near the Centertown building, the main pool, and a bus stop would be (in order);
Buildings 26, 25, and 24 in the Martinique section
Buildings 31, 32, and 33 in the Trinidad North section
Buildings 45, 44, and 46 in the Jamaica section
Buildings 51, 52, and 53 in the Aruba section
The last two sets of buildings (Jamaica and Aruba sections) are actually on the other side of the resort's waterway, but only are short bridge walks away from that Centertown building.
I am glad to field any follow-up questions that you may have!
Have a magical vacation!