Hi Coleen
You fire "Get Activity Types" API.
You can get all types and fields in each type.
http://developers.marketo.com/documentation/rest/get-activity-types/
Example
{"id":1,"name":"Visit Webpage","description":"User visits a web page","primaryAttribute":{"name":"Webpage ID","dataType":"integer"},
"attributes":[
{"name":"Client IP Address","dataType":"string"},
{"name":"Personalized URL","dataType":"boolean"},
{"name":"Query Parameters","dataType":"string"},
{"name":"Referrer URL","dataType":"string"},
{"name":"Search Engine","dataType":"string"},
{"name":"Search Query","dataType":"string"},
{"name":"User Agent","dataType":"string"}
]},
{"id":2,"name":"Fill Out Form","description":"User fills out and submits a form on web page","primaryAttribute":{"name":"Webform ID","dataType":"integer"},
"attributes":[
{"name":"Client IP Address","dataType":"string"},
{"name":"Query Parameters","dataType":"string"},
{"name":"Referrer URL","dataType":"string"},
{"name":"User Agent","dataType":"string"},
{"name":"Webpage ID","dataType":"integer"}
]},
{"id":3,"name":"Click Link","description":"User clicks link on a page","primaryAttribute":{"name":"Link ID","dataType":"integer"},
"attributes":[
{"name":"Client IP Address","dataType":"string"},
{"name":"Query Parameters","dataType":"string"},
{"name":"Referrer URL","dataType":"string"},
{"name":"User Agent","dataType":"string"},
{"name":"Webpage ID","dataType":"integer"}
]},
:
:
: