Being my first travel tip, I'm going to take it slow. The way travel should be!
Tip number 1: Walk (if you can)
Obvious yes, but not everyone does this!
I find walking around cities I'm visiting an enjoyable way to see the city. You never know what you might discover at a walking pace (rather than drive, bus or bikes). It also makes it a lot easier to take photos!
Reasons it's sustainable:
1. Minimal impact on the local environment (as long as you're respectful if walking through parks and don't litter!)
2. Gives one more opportunity to observe and respect the local culture
3. More chances to spend money in the local economy by coffee and cake <insert your preferred travelling pastime here> stops.
Travel safe!