Depends what they're into.
Drama
Acrobatics
Board games/card games club
Cooking classes
Doing something they already enjoy a bit will be much better than a brand new activity clearly picked for them as forced social interaction. Kids are smart.
Has this child been in school and not getting enough social interaction or support from teachers to interact more confidently? Do they avoid interaction in ALL settings? If in school and truly an issue (e.g., not participating in small group work at all, wandering off to be alone every recess and hard to get to re-join the group), ask for a meeting and request testing. Hearing, articulation, sensory, attention, and language processing issues are just a few of the common things that can be identified and supported.
But if it's only a few days into a new school year, and your child hasn't warmed up yet to new teachers and classmates, then let it be
