What can I make out of a wooden headboard besides a bench?

Dependent on the Quality of the Wood and your access to a Table Saw (Many High Schools have these and will cut down boards for a fee or donation, you must check with the school) you can make a Variety of Products or Facing, Just like any other Stock Wood or Trim.

Many designs for Shelves or Mantles look much better than a Bench, guaranteed.

you can make a storage area for shoes and add a back for hooks to hang coats. It can be put in garage or in an entrance to your home for family to use. If you have children, they can put there book satchels,shoes and coats or toys.

Coat rack for foyer or anywhere.

https://www.diyncrafts.com/35434/repurpose/20-astonishing-repurposing-ideas-old-headboards-footboards

Here are instructions for making a sign out of a headboard... https://www.homeroad.net/2016/05/repurposed-headboard-sign.html

A rolling caddy with wire baskets for your kitchen. Top being the headboard, THIS pic doesn't have the bottom piece but could be added, legs being 2x2's, wire baskets that hang over cross beams supporting the legs & casters on the bottom (then painted). Here's what the finished product could look like .......