Ideas:

The Norman Horseman - what he would wear, how he would ride and fight, etc.
A Tournament Cheque - A scoring sheet based on a x-century tourney cheque
Renting a Lang's Horse - What to expect and what to do (Lang's specific)
A Senechals' Guide to Equestrian Activities - What to do with them horse people, their events, insurance, the waivers, oy!
Making weapons (choose your type)
Improving your game score: (pick a game you do well or give a different perspective)
Invading Hastings - Describe the Hastings event and how cool it is.
Book reviews are always fun and useful.
A description of a tournament that would have been held in your period.
Developing a GREAT relationship with your rental string owner

Designing a challenging, challenge course

A survey of the equestrian classic riding authorizes from Xenophan to Pluviel - who they are, when they are, what the highpoints of their schools, and resources for them (Violante)
A timeline of highpoints in equestrian stuff.

A glossary of terms, including weapons,

A Baron and Baronesses introduction to Equestrian Activities.

Tips and Tricks for Photographing Equestrian Activities by Layla.