There's obviously the AJM 9 RoboCop gun, there's an Indiana Jones reference in a cave with tons of "grails" everywhere, there's the "arrow to the knee" guy...
I don't remember the exact locations but they are there. Pretty sure there are lots more to find.
One thing that keeps bugging me are the lapel pins. Are these things also a reference to something? I keep getting that impression since their are so many of them.
I guess this has been mentioned before. During the opening cut scene when the bus is stopped and shot up. Noticed that the selfie is a mirror image not a true selfie.