I usually put on goggles around 15F because otherwise my eyelashes ice over (I call them eye-cycles) and eventually I can't see.

You said sandals were an option. I wear sandals year around because I can layer socks as needed. If you go this route, just be sure the sock layers don't constrict your toes or you'll sacrifice insulating properties.

You probably already know this, but if you'll be in the cold for an hour try to regulate heat early on so as to avoid sweating a lot. You don't want to get wet in the first 10 minutes and then have to ride that way the other 50. It can be easy once you start to warm up to forget to ventilate until it's too late.

Good luck.