Hola! I'm another car person, who mothballs my precious baby for 6 - 15 months at a time. Here are a few things I've learned the hard way.
1. Battery. In my car, disconnecting the battery = non-functioning door locks. Neither the key fob, nor the manual keys will work.* That means I can't pop the bonnet to get the fucking battery reconnected. I simply leave the driver door unlocked, but that could make some people uncomfortable.
2. Pour some gas stabilizer into your tank. Three months probably doesn't matter that much, but gas can go stale.
3. Fill your gas tank to the tippy top, to discourage water. (caveat: advice on this varies. It's something I do, but others oppose it vehemently)
4. Take out your air filter! Mice love, love, love to nest in it. I blew those pink squirmers and their mommy's poop into many places. It was gross.
5. If you're really going to a while, consider jack stands. I've seen guys return from deployment to square tires. It's a very sad moment for them.
6. Consider removing you wiper blades and storing them inside the car. One time mine glommbed onto the glass. It was gross.
7. Manual cars, release the handbrake, leave the car in neutral, and chock the wheels. Break shoes, etc can rust in place.
8. If you like your paint job and don't have a garage, consider a cover. Off the shelf are okay, but I bought one custom fitted to the car and have never looked back.
*Because I have a play car. The windows have to roll down slightly before the door can open. This can't happen without power.