Once you're an adult, learning a second language is as difficult as playing the guitar sounds (does not apply to fourth, fifth or sixth language).
I like the volunteer idea, but it requires going somewhere with a little one unless you have a sitter.
I really like the idea of make-up tutorials on YouTube.
But it depends on what you feel is lacking - social stimulation, mental, physical, spiritual, creative (craft, cooking).
I'm not a new-ish mom anymore, but here are a few things I've done in the past few years that qualify:
ballroom dancing (my son was 1, so that was awhile ago)
swimming
intense uphill walking
rowing (machine)
martial arts/self-defense
meditation
helped with a school board campaign
bar/bat mitzvah committee
knitting
watercolor painting
book club
library school