The handling skills a weanling learns in its first year are the vocabulary it will use in every subsequent training context, and the trainers and riders who inherit a well-handled weanling consistently describe the difference as night and day compared to horses that were neglected in this period. The most important skills to establish are the ones that make every future handling interaction safer and more cooperative.
Haltering and catching rank first — a horse that can be caught easily and haltered calmly by any competent handler is safe in any situation that requires restraint or examination. Leading correctly — walking off immediately from light pressure, stopping softly, maintaining appropriate space from the handler, and not pulling back or running ahead — is the foundation of every other handling interaction. A weanling that leads well is safe to take anywhere and easy to manage in any situation.
Foot handling and accepting each foot being picked up, held, and worked is essential preparation for the farrier and veterinary care. Accepting grooming tools, spray bottles, clipper sounds, and being approached from all angles by people carrying equipment prepares the horse for the inevitable novelty of training and daily care. Standing tied patiently without pulling back makes routine management safe and efficient.
Beyond these core skills, loading into a trailer — even if only for short practice trips during the weanling year — is enormously valuable because it is a skill every horse must have and one that is far easier to establish when the horse is small, light, and manageable than when it is a full-grown adult. A weanling that has been walked in and out of a trailer ten times during its first year will load as an adult as if it has done it a thousand times.