1. Strength. Whether it's lunging or long walks through the woods - actually I hope you are doing both. If you want to come out in the spring and pretend that you and your horse didn't lose any dressage skillz, then I would focus on strength on those rainy days. I'm sure your horse is smart, and competent, and knows what you want him or her to do. If I have learned anything in the last year it's that most techniques boil down to strength. Your horse KNOWS how to go sideways, but are they strong enough to do it? Your horse KNOWS you're asking them to slow that canter down, but are they strong enough to do it? LIKEWISE, if you're not practicing your sitting trot every day and planning to debut at second level, I hope your hitting the gym with weight training a few times per week to keep your core strong.
2. Turn on the haunches/walk pirouette - This is one of my favorite exercises to practice in the winter in the barn aisle because it's pretty much my only option for turning a big horse.
3. Square halts with clicker training. I confess, my horse knows when I have candy in my pocket and she is magically square when she knows that I am carrying treats. I think it helps to positively reinforce a square halt because it's really kind of an arcane concept that most horses need a jackpot to understand why you are nitpicking their feet. I like to practice my trot-halt square-trot on rainy days so that I am not fretting about it during show season.
4. Turn on the forehand. How do you make a stinky mare listen to your leg without being mean? You remind them very gently with a turn on the forehand. Works like magic!
5. Quarter turns - think about it...you're asking your horse to load that inside hind leg on each turn. What a great way to ask them to bear weight and get stronger!
6. Half-steps. I bust out the clicker for half-steps and it motivates my horse. Sometimes I tap certain legs to ask my horse to lift it, other times I work in hand reinback to trot- but I can't think of a more fun rainy day activity.
7. Reinback - Thinking again of strength and working that hind leg - asking your horse to reinback is a great exercise. Also you can combine it with fun variables like reinback to canter (and you pick which lead) or reinback to half-steps.
8. walk-canter-walk - How many canter strides can you fit down your barn aisle? I like to work on my walk-canter departs out of a quarter turn to help set my horse up for success to pick up the correct lead.
I hope this gives you a few ideas for the next time it rains!