You could try transferring back and forth with Marriott (which both sites text say will NOT renew your expiry IIRC) ... but ... try it anyways to see?
Otherwise get an SPG Amex for $120 - $30 GCR (or a kickback from a friend referring you) and earn some SPG on your spending.
I actually keep the card as my daily use card since I value the earnings at about 3% on everything.
ps. You can also book a room on points and then cancel it and get the points refunded... but that means you need enough points to book a room so it won't work for small leftover balances.