I like to set a lot of my groundwork for the following year during the dormant season. Making soil, repotting, retagging, laying weed fabric, updating my growlist, and most importantly buying new plants! Now that I type it, I imagine it actually sounds really boring to you folks, but hey. Keeps the stir crazy at bay!
And the winter-growing or tropical stuff is a definite must as well. Need planty playthings in winter or I go mad.