Hey Tim,
Sorry about the super delayed response, I just recently took over development duties on the API. The issue you're having is that the API currently only allows specific values for Activity types: type_other, type_call_talked, type_call_lvm, type_call_missed, type_email, and type_meeting. This is definitely not explained anywhere, and I'm sorry for the confusion it has caused. Within the next couple days I will be pushing a fix that adds proper validation to that field which will return the list of allowed values to prevent future confusion.