It's just over a year since I lost my Dad, so I can understand how you feel. His birthday is in December too, though he always hated anyone making a fuss about it and we haven't planned anything special.
It sounds like your Dad enjoyed nature, so perhaps you could plant a small tree or shrub in your garden (if you have one). I think it's the right time of year for planting things.
My husband's birthday was on Christmas Day, and he died 2 days before, last year. I've helped to sponsor the floodlighting at my local church for the month of December, and I have planted a shrub (Campsis) for him.
I have an entry in the Book of Remembrance for our Wedding Anniversary, and I planted a rose in the garden, after his ashes were committed. On the anniversary of his death, I am going with a friend to see Sheridan Smith in Funny Girl - this was the only day I could get tickets for, so I think it was meant to be.
I'll be on my own on Christmas Day, remembering his birthday, and will drink a toast to him. I'm a sentimentalist, and have kept all the cards we've sent each other, and I sorted these into different folders, so I'll have nearly 50 Christmas cards from him to read through.
We've just all got to get through these days as best as we can, and I hope others succeed.