I used to play with my chewing gum all the time, but I quit gum 10+ years ago (after i quit smoking) so started making / wearing bracelets to twirl and twiddle with (beads on elastic ...not sure what i like best; that they're cute or that they're great to play with). I only tend to wear them in spring / summer tho, so other times i tend to resort to finger fidgeting or hair twisting.
I always carry a puzzle book on eg trains, so i have something to look at and can ignore all those people looking for eye contact. Of course, sometimes it's too crowded to concentrate, and if it's too smelly or people are jostling me i might end up stimming more 'obviously' ~ twisting my hair, tapping my temple, 'flapping' my hand-wrist, and / or rocking (as discretely as i can!).
If i'm around another person who stims often enough, i might pick up one of their 'tics' too, but that doesn't happen often, or last long ~ the only body movement i know i've picked up from someone that has lasted is the yes/no head movement one of my favourite indian actors does sometimes;
one day, someone nice asked a yes / no question i wasn't sure how to answer and it just happened ...and felt really nice (!) so it kinda stuck.