What about in hand classes (grooming, turnout, costume) and shows for the off beat disciplines (polo, drafts, minis, handlers under 9 years old, side saddle )?
Clinics from a vet, a chiro, equine massuese, show groom, maybe a local judge, etc.
Do not take anything to heart. Do not hanker after signs of progress. Founder of theRiders with Fibromyalgia clique.
Pony Parties in which you bring a pony to a house, or you rent your facility for a few hours
Camp during school breaks
'Camp'/ horsemanship classes for adults (check w/your local YWCA, or Continuing Ed)
Educational seminars in which you hire a pro (ie: farrier, vet, nutritionist, etc) and charge an attendance fee in which the pro covers a specific topic, then finishes w/Q&A. For more revenue, provide refreshments and charge a markup. Or get really fancy & hire a caterer and have a sitdown dinner.
I'm doing a Black Friday camp this weekend and parents are always looking for something for the kids to do during winter break. You'll get alot of lesson students from camps.
Also you might try a community type event. We've done those at my barn a few times. Fall and spring festivals with pony rides and bake sale, maybe a bounce house if you have the room and your horses can handle the noise.
I started running home school classes (teaching P.E., science, and history with horses) at my bosses barn, not always riding, sometimes just working with the horses or doing projects related to horses (discovering horse genetics, or getting hay and grass tested, or picking a period in history that was influenced by the horse).
It was a HUGE hit with the local homeschool community. Something fun and different. I did 1 hour/week for between 6 - 12 weeks depending on the class. We usually had 6 kids in each group, and I had an assistant (homeschooled rider at the barn).
Between that and the "Women & Horses" and "Adult Horsemanship" classes that I did - oh, and "Kinder-Ride" - 1/2 hour lessons for 4 - 6 year olds on SAINTLY ponies... usuallly 15 -20 minutes off the horse and 10 - 15 on - I made my boss an extra $25,000 + in about 5 months...
Think creatively about getting your currently clientle involved in more ways and finding new people, even if they're not people who will be involved for ever... the adult riders came for an 8 week class and some kept riding once a week in their group, but not all of them... and then we'd get a new group in.
There are some terrific ideas here, but I would caution the OP to check their insurance policy. Some carriers won't cover specific activities unless additional premium is paid and some will exclude specific activities altogether.
I also agree that thinking out of the box is critical. I would look for home schoolers as they often need a PE requirement and can come during the day. The suggestion of working with a dog club for obedience and agility training is a great way to get some money out of that indoor ring.
There is a local barn here that targets the homeschool crowd. I don't know the in-depth details, but basicly they offer a 2 and a 1/2 hour time slot, to 6 kids at a time, that covers grooming, tacking up, cleaning tack and of course riding. The kids also get an education about the other animals--bunnies, goats, etc. What I do know is that they have 2 sessions a day, 5 days a week and the homeschool parents pay about $60/month. That's $3600 a month of extra cash for the barn, for utilizing daytime hours that are generally quiet. There are about 60 kids in the program, and I'm pretty sure they have a waiting list.
I should emphasize that this is not "play time"--the barn owners are very thorough in getting educational materials for the kids. But they do make it fun and it seems to be a big success.