In case you haven't managed to repair the Greenhouse by your first Stardew Valley winter, get your hoe and start digging for snow yams and winter roots all around Pelican Town and its surrounding areas. Don't forget that you can also find Crystal Fruit and Crocus along the way by regular foraging.
You will have the crafting recipe for Wild Seeds (Wi) by winter. Throw together a snow yam, winter root, crystal fruit and crocus to make 10 Winter Seeds. Alternately, put a winter root in the seed maker to get up to 3 of these mixed seeds.
You can plant these and see what will come up. Junimos do not harvest crops during winter. Additionally, you do need to water crops during this season, even on snowy days. So unless you have sprinklers set up, it's not worth the effort in terms of returns. Also, fruit trees don't grow in winter.