Last Contact/Activity Date

I have been asked to report on the last activity date of an account from a range of subject areas.

I was wondering if anyone had any recommendations on how I can achieve this.

My idea was to create a workflow so when an activity is completed a field is updated at account level with the data and contact method but i am not sure how I can update the field from a workflow that is triggered from a different record type.

Do you have any ideas? 

Thanks