You can exchange the Marriott points for Lowes GCs. Check with your local Lowes to see if their GCs can be used to purchase other GCs. They often have Disney GCs.
Marriott Rewards also has Staples GCs. Sometimes Staples carries Disney GCs in their gift card area. You might be able to convert Staples GCs to Disney GCs.
I don't know anything about United, they're not a major carrier here and we don't fly with them. However, there are websites (mileage exchange, points.com) where you can sell your unwanted FF miles. You'll take a steep hit on the value of those miles, but you will get cash for them. I've never tried these, although I did become a member at points.com years ago.