Although someone with severe-stage dementia may seem beyond all interaction, you may be able to reach in and connect through smell. (The sense of smell tends to diminish with dementia, but many people retain some of it.) Memories connected to odor tend to be deeply enduring, like Proust's memories of madeleines. At minimum, familiar and pleasing odors can create a happy mood.
Some things to try:
Perfume or cologne. Know your loved one's favorites? (Hint: Look in dusty bottles on the dressing table or in a medicine cabinet.) Spray a little Old Spice or White Shoulders in the air or on a wrist. Older women may favor dusting powder.
Work-related scents. Ex-teachers may respond to chalk or crayons, farmers to fresh-mown hay.
Strong, distinct flowers. Hyacinths, lilacs, and roses are examples. Did/does your loved one keep a garden? Bring in fresh-cut flowers or find a potpourri to place in a bowl (if you can be sure your loved one won't attempt to eat it).
Lotions. There's no end to options, from citrus to vanilla to musky scents. Explore a drugstore or chain selling inexpensive lotions and see if any remind you of your loved one. Or mix it up; buy several small bottles and use a different one on your loved one's hands each day.
Spices and herbs. A cook may be drawn to cinnamon or nutmeg. Open a container to sniff, or shake some onto a potpourri. Or keep a plant of fresh basil or lemon balm handy.
Other homey food smells. Bake some fresh cookies, cut lemons, open a bar of chocolate. Even someone who no longer likes to or is able to eat these things may spark to the scent.