For creating activity you may go to rizecrm_crmentity_seq,this is id of last created record,note this id.
Now go to rizecrm_crmentity.insert new record in this table,you may notice a field crmid, put the value an increment of 1 from the id u noticed from rizecrm_crmentity_seq (for ex,if id in rizecrm_crmentity_seq was 1,you need to put value of id as 2 in rizecrm_crmentity).
There is another field setype in rizecrm_crmentity,put the value as calendar in this field,label is the activity name.(you may fill other fields as per your data).
After adding record here move to rizecrm_activity,put crmid value from rizecrm_crmentity in activity_id,add other fields,after this in table rizecrm_activity_cf,put same value of crmid in activity_id.
If you want to relate this activity to organisation,contacts and other entities, just move to rizecrm_crmentityrel,put entity id(lead,contacts etc) id in crmid fields,in module field put
the name of that entity(for eg. you want to merge lead put the value Leads in this field),in relcrmid put activityid in relcrmid and calendar in relmodule.
Lastly in table rizecrm_seactivityrel just put module ID in crmid, activityID in activity.
Now go to activity tab inside that particular module, you may find activity attached to that entity.