Upvote:3
Your cheapest and simplest option is to get a passport card. It costs $65, is valid for 10 years, and you can apply at a post office or similar location near your home. It will probably take 1-2 months for the card to arrive, so you have plenty of time.
However, the passport card only allows you to travel into Canada, Mexico, and certain Caribbean nations by land or sea. It will suffice for the trip you have in mind, but if there is any significant chance that in the next 10 years you may want to travel internationally by air, or to travel beyond North America, you may want to just pay the extra and get a regular passport instead ($145).
Global Entry does not help because you have to already have a passport in order to apply for Global Entry. As for NEXUS, if it is anything like Global Entry, they will only mail the card to your home address and it takes several weeks to arrive.
You are correct that a birth certificate and driver's license are not sufficient to enter Canada and come back to the US. And California does not issue enhanced drivers licenses. As of this writing, NY, MN, MI, VT and WA are the only states that do.