I'd go for 'consumables' rather than things. So think of literally anything you know they like and buy posh mini versions. Eg. some stuff I'd like would be…
Jo Malone travel candles
Fortnum and Mason jam, marmalade, chutney, Christmas preserves
Umami paste, gentleman's relish, olives, tomatoes, truffle oil, vinegar etc
Cashmere socks from TKMAXX
A personalised Christmas ornament each
Seeds for the garden, plant food, self watering devices
Aldi or Montezuma chocolate buttons
Nuts, pork scratchings, dried fruit etc
Alcohol miniature
Travel size perfume
A nice soap or hand wash
Liberty print face mask
Bottle of matches
Maldon sea salt
Truffle powder
Waterstones gift card
Cocoa dusted almonds, sugar almonds, candy kittens
Chocolate biscuits
A set of postcards and stamps
Gel writing pens
Zara home Reed diffusers
Coffee / Beanies coffee
Spanish hot chocolate
Zoflora
Make up bits
Bath stuff or bath bomb