I think a photo will be necessary to answer your question.
Depending on the size could you mount it on a small table and use it as a mailing center? inside put stacking "in and out" baskets for bills and payables, stamps etc. A unique way to keep your mail together in one spot
I have used one to store my small garden tools outside.
A photo of the mailbox would helpwith ideas. If it is a "tube"type, you can fill it with fake flowers or use it to hold outgoing mail, or inthe winter...hats, gloves and scarves.
If it is a lift top type, use it for mail, a drop spot for keys, phonesand wallets. Or use it as a planter boxfor your favorite herbs in the kitchen or for bright flowers on the porch or atable on the patio. Lots of possibilities.
Paint a rural landscape on it!
Paint a rural landscape on it
I'm not sure what it looks like but maybe it would be cute mount it near a desk or on the side of a kitchen counter to hold actual mail or office supplies.
Bread box maybe.
I hung my old beat up box vertically on a wall and put plants in it.
Magazines. Gloves & hats. Cat bed ( brace the door so it doesn't fall shut.)