Normally tourists visit Kochi, Munnar and alleppy, it's nearby districts and easily reachable spots within short period. That will be better if you don't have more than 5 days to stay in Kerala.
Depends on the no. of days you are staying.
Alleppey. Munnar. Kumarakom. Wayanad. Kovalam. Kochi. Vagamon. Thrissur if you want to do road trip, then you can also go for Bandipuram forest to Bangalore by road.