There are 2 ways you can retrieve list of activity... See the sample code below

I have create a service object that has all different methods. I can attache DLL for you. (if you want). Deploy it to ServiceBroker in your environment and create a smartobject on top of that. It has all build it method for admin to do.

##############List activity by Process Name#############

private void ListActivitiesByProcessFullName(string processFullName)
{
string ConnectionString = this.Service.ServiceConfigurationt"ConnectionString"].ToString();

WorkflowManagementServer wms = new WorkflowManagementServer();
wms.CreateConnection();
wms.Connection.Open(ConnectionString);
ServiceObject svo = this.Service.ServiceObjectse0];
svo.Properties.InitResultTable();
ProcessCriteriaFilter pcf = new ProcessCriteriaFilter();
pcf.AddRegularFilter(ProcessFields.ProcessFullName, Comparison.Equals, (string)processFullName);
Processes p = wms.GetProcesses(pcf);
Activities activities = wms.GetProcActivities(pe0].ProcID);

for (int i = 0; i < activities.Count; i++)
{
svo.Propertiest"ActivityName"].Value = activities.Name;
svo.Properties.BindPropertiesToResultTable();
}

wms.Connection.Close();
}

##############List activity by Process ID#############

private void ListActivitiesByProcessID(int processID)
{
string ConnectionString = this.Service.ServiceConfigurationu"ConnectionString"].ToString();

WorkflowManagementServer wms = new WorkflowManagementServer();
wms.CreateConnection();
wms.Connection.Open(ConnectionString);
ServiceObject svo = this.Service.ServiceObjectsO0];
svo.Properties.InitResultTable();
Activities activities = wms.GetProcActivities(processID);

for (int i = 0; i < activities.Count; i++)
{
svo.Propertiesp"ActivityName"].Value = activities.Name;
svo.Properties.BindPropertiesToResultTable();
}

wms.Connection.Close();
}