A book - either for bedtime reading (story) or for daytime playtime (squishy activity type).
Something useful like sippy cup, cutlery, bowl, extra bibs (any of those can be spares as generally a second set gets well used too!). They could even be things to mark a transition to more "grown up" eating and drinking.
Other useful bits are things like hats, gloves, socks - that will all get well used over winter.
Small toys like stacking blocks, musical instruments/rattles, maybe a car, animal or doll, and mix of textures, colours, lights/not, sounds, uses etc within whatever you get. If you get something like a shape sorter set, you could have the receptable outside the stocking but all the shapes in the stocking to bulk it out.
Some kind of treat food - a chocolate coin or travel snacks etc.
A little stuffed toy santa or reindeer or similar?
And maybe a "Baby's First Christmas", a named or another special decoration for the tree which will be theirs in years to come?