Equipment maintenance, clean-up, change plugs, oil, filters, air filters, sharpen or replace blades. Grind rust and paint everything possible. Lubricate everything possible. Change diaphragms in pumps, o-rings in guns. New gears in spreaders. Sometimes re-calibrate spreaders and sprayers for better accuracy. Make sure everything is working perfectly. Go to turf conference. Shop for next year's product allowing several dealers to show me their programs, discounts and payment plans.
Retired now, but I normally made up a new marketing plan. Find green grass photos and prepare mailers, flyers and estimate folders for next year. Try to take advantage of any year-end, or early-bird sales, (equipment, uniforms, fert, herbicide, printed invoices, envelopes). Learn and improve software.
Finally, I always wrote my newsletters in advance. I had these printed in January and passed them out at each round. I included information on watering, mowing correctly, earth-friendly garden care, grubs and pests to watch out for, and optional (upsell) treatments they might want. Always mention your customer referral plan and the rewards of same.
I never had a website--but winter is a good time to build or improve a site.
Its also a good time to drive down major streets in your favorite area and and compose a mailing list of potential customer homes. Use it two to three times per year--year after year. Prepare and address letters and stuff the envelopes in advance in January.