I have few suggestions and I have been to all.
If you want to do Central America go to Costa Rica fly into San Jose rent a one way car to Liberia but before returning the car stop and spend night at Manuel Antonio and arenal volcano. Also after returning car you can go to Nicaragua but for single lady I don't know if it is too safe.
Best way to use your 2 weeks will be Vietnam and Cambodia. I did 10 day package tour from local tour company and you visit Hanoi Halong bay hue ann Saigon then take a river cruise in nmeekong delta to Cambodia arrive phnom peh and go visit Angkor watt. The fly to kula lumpur and Singapore and your 15 days will be over. The 10 day package tour was $900 per person including hotel site seeing guide car and transportation.