for sure a first aid kit, go to your local tack store and there should be a product called blue lotion. you put it on small cuts or scratches and it prevents them from getting infected, lymphingitis and other related things and i would also recommend lots of vet wrap.
Make sure you have all the equipement required to ride the horse (saddle, briddle, saddle cloth, boots if needed, ...). Then if the horse requires some special food or hay, be sure to have enough of it.
I suggest you to have a little first aid kit in case of a minor injury. Hope this helped you :)