Worth it: a proper rain cover (British weather doesn’t negotiate), a footmuff for anyone using the pushchair through autumn or winter, a buggy board once a toddler tags along on the school run, adapters if you’ll switch car seat brands later, and a decent changing bag that actually clips to the handlebar.
Usually skippable: branded cup holders that only fit one specific handlebar shape, matching parasols in climates that are more rain than sun, and organiser pouches that duplicate what a normal changing bag already does.
One that surprises people: a simple stroller hook (a couple of pounds) does almost the same job as a branded organiser for hanging shopping bags, at a fraction of the price.