- different types of transport (tube/bus/train/taxi/tram/cycling)
- different types of tickets and cost (weekly/annual/return/round-trip)
- concessionary fares (child/student/OAP/disabled)
- peak time and off-peak and the difference between them
- public transport etiquette (probably depends on what country you are in - I see you are in the US but here in the UK it's very much "not done" to make eye contact or conversation)
- map-reading/ getting directions/ using a satnav (could be a great listening practise!)
- road signs and the highway code
A lot will depend on their level of English and their native countries, because some things will be very familiar and they will simply need to learn the English word, but other concepts may be totally alien. Good luck with it!