Not a trail rider, I do dressage, but we also get bored just going around in circles!!
I've taken a liking to doing a lot of bomb proofing, all you need is a plethora of scary objects (tarps, exercise balls, etc..), obstacles and patience!! I've also started teaching some tricks, starting with bowing, standing on things, etc..
Dressage is also fairly easy to start with your horse on your own. It is basically about improving the response from your horse to your aids and involves being able to place the horse in the gait you want, at the speed you want with the touch of a button. Leg yielding, turn on the forehand and turn on the haunches are also good exercises to work on with your horse so you can place his forehand and haunches where-ever you would like. Lots of transitions are also helpful in putting the horse "on your aids".
They say money doesn't buy happiness -- well happiness doesn't buy horses!