Are you tired of passive solar ideas yet? Just one more...
A good magnifying glass is worth every penny. When you need to start a fire in a fire pit or campsite, use the magnifying glass to concentrate the sun's rays onto a piece of paper or dry leaves. It won't be long until a flame appears then you can add other fuel carefully.
If you have a fireplace or woodstove, you can even start a fire outside in the sun and bring it in. Just use a wide mouth metal container like an empty tuna can, poke a few holes around the sides for air flow and put the paper or other tinder in it. Use a cloth or create a handle of some sort to move the can of fire indoors.
It's no wonder that early religions worshiped the sun. We owe much to this ball of fire and we could use it a lot more than we do.