Now, there are a few options...
1) Bring them inside and let them ripen, but they won't taste quite the same;
2) Fry them up, fried green tomatoes are delicious but there is only so much of that you can eat; and
3) Make preserves, it uses a good sized batch at once and they're an excellent homemade gift.
We did a combination of all three, but the winner for long term storage and enjoying our garden through the winter would be #3 Making preserves. I decided to use a combination of apples, green tomatoes, and our cowhorn chilies to make a sweet and spicy chutney perfect for papadums, dosa, or use on a cheese plate (It's great with smoked Gouda! :)).
This recipe is for 2 pints of this sweet, and spicy, spiced chutney.