Rent Rooms, another unit, Airbnb or storage.

So another option is to put a permanent window seat and have some storage shelves build in to make it look like a feature rather than a cheap fix.

I called the city inspector and asked if I could add the rooms, including closets, and call them storage rooms when I list the property. 

If you do find something that may have value, I recommend you place it in storage (even if it's your garage) and wait a couple months to see if they come for it.Regardless of what you find, it is important for every landlord to understand what the law says regarding an abandoned property and anything left behind.

Can I remove it and put in a basement or other storage?

Our current apartment is very cozy/convenient, but with our additional storage, totals almost $3000/month.

We were in your shoes once and found creative solutions!

I was in your shoes not too long ago. 

Boat storage is also a zoning issue, need to be zoned appropriately.

We would simply install a piece of storage furniture called a chifferobe (a free-standing closet) and that would settle the issue.