Originally Posted by Grinnin
You mentioned oil change. I'd also
- Put stabilizer in the gas tank and petcock
- Drain the carbs
- Remove spark plug and spray in some light oil
- If the bikes have a clock or other always-on drain, disconnect the batteries
- Charge up the batteries once a month
- Put steel wool in muffler exits and airbox intakes to keep out nesting critters.
Modern tires don't flat-spot like tires in the past, so hanging in the air probably isn't required. And I, personally, wouldn't just start them to let them idle, but wait until I wanted a ride and work on them then.
This is just a partial list, I'm sure there will be more.
Great list. Consider replacing the GF, it's sure to be an ongoing issue left unattended.