A coffee table - place the slides or negative in between two sheets of glass, light it from below and insert it in a coffee table frame. if you use the mini negatives you could have a magnifying glass around so people can fish through the collection.
A wall feature - Same as above really, glass, light from behind, attach to wall. Leave out the coffee table
Jewelery - cut out little images from the slides / negatives and encase them in resin and create jewelery - preferably earrings as they are more likely to have light shining through them
Coasters - stick the slides on white backing and level / seal the top with resin.
Curtains - like those beaded curtains, you could string the slides together and use them to decorate neglected doorways, doors and walls.
Decorative bunting - again string the slides together and use them to decorate at you next family event.
Graffiti gifts - find some nice slides picturing people places and graffiti them with moustache, eye patches and silly hats. Then give people their 'new and improved' image along with their Christmas gift.
Use the slides as gift tags - just write on the white borders.