1st thing is the steps and flooring in the attic - we use ours for Xmas stuff and grandkid stuff and a queen size airbed in its bag. Not stuff you need frequently - those steps are kind of dangerous.
Next - go for the 3rd bedroom if you can, not a den (with no closet) and customize the closets, either yourself if you're able or use a closet company. Try to figure out what you want to store there first - crafts, files, games, wrapping paper, whatever. We have volume ceilings and 3 shelves above hanging clothes - with the top shelf being wire again so we can see what's up there (old people tend to forget
. We keep mostly luggage and a plastic bin with snorkeling eqpt for cruises. You don't want to get up there too often either.
If you have an inside laundry, this provides another room of storage - add cabinets over the washer/dryer and shelving on the other side if you can. We have a shelf above the 2nd fridge and laundry room sink and a small shelf opposite next to the washer/dryer cabinets for bleach, white vinegar, distilled water, etc. It's fairly high - we used leftover wire closet shelving so I can see what's up there.
We also changed out the bi-fold door on our pantry for a regular paneled door and hung shelving on the inside of the door for lightweight pantry stuff - cereal, etc. Frees up a large shelf for small kitchen appliances so I don't have to bend down and poke around lower cabinets.
Don't forget about under your beds - we keep a crib mattress for the preschoolers in our family, and maybe extra winter clothes until you figure out how much you are going to keep. That may depend on if you go back at Xmas, etc.
The garage is a whole other chapter and there are multiple ways to do it. I'd suggest driving around and stopping to talk to guys puttering around in their garages - most would be happy to share tips.