Dropping enemies by non lethal means will always give you a nice bit of bonus XP, be it stun gun or tranquilliser, while stun takedowns (activated through a tap rather than holding the button) will also net you tasty bonus xp.
If you’re trying to log into a low level computer device it’s always a good idea to hack it even if you know the code as this will give you some xp you wouldn’t otherwise get.
‘Traveller’ xp is some of the easiest to find, so it’s worth taking a short crawl up any vent you see as you’re more than likely to be gifted a nice bit of bonus experience.
It’s not just vents that will grace you with xp, when in Detroit and Hengsha make sure you try and take a trip onto the rooftops as there’s explorer experience bonus’ us there.
This may be sounding a bit old already but honestly, look everywhere. Jump over fences, look down dead ends, there’s bonus XP hiding in all the corners of the world, from explorer bonus’ to eBooks, you just have to make the legwork to find it.
Whenever you’re given a secondary objective, try your hardest to complete it, they often come complete with a healthy serving of experience so just try will you?