For a toddler, things they need anyway:
Small sized book, especially a seasonal one, but even a Mr Men, Thomas the Tank Engine, hardbacked book etc.
Crayons or pencils and a mini-colouring book or plain paper notebook
A Christmas plastic glass (older DC could have a plastic Christmas mug) to reuse every year for December (DD is still using hers over 10 years later, and it gets put out for Santa annually also despite us all knowing she no longer believes, the ritual is still important! 
)
Or on a similar line, an age-appropriate water bottle for bringing out and about. Could get an age-appropriate one for older DC also.
Packet of "their" treats/snacks - raisins, dried apple, liga biscuits - whatever it is they like.
Hair bobbins etc, new socks, hat , gloves, ….useful small items of clothing
rubber ball or small car or small doll etc
Places like Ikea and Primark sell smaller fleece blankets in nice colours patterns that could be folded up small and take up space
Some DCs love the simple things - I know I've seen people here talk about a large box filled with balloons (already blown up) being a great hit with toddlers!
Small soft toy