I don't know your horses temperment but one of my favorite things is to get on mine in the arena or roundpen (usually bareback and bridaless), close my eyes and keep her walking. Doesn'y matter where she goes just that she keeps walking. The warmth of her body coming up through me, the gentle swaying motion, the sound of her feet on the sand and her hay breath.... even if you can't ride try to meditate on the experience of riding by bringing up all the different sensory parts of a ride. And (short of lightening) a ride in the rain can be really great too... taking in all the wonderful senses of riding and the clensing feeling of the rain coming down on you washing away the crud of the day. I also have a bracelet and a key chain made from my horses hair (just a little clip from under the tail and braided...) I hold it and feel it as I meditate on riding when I can't be with her... It also has a really nice side effect... my riding has improved!