if you like dogtown and the z-boys check out 'skateboard kings' the world in action documentary about the scene from 1978. its on youtube.
there is a uk skate docu called 'rolling back the years' that is a bit longwinded but fascinating to see the uk skateparks of the 70's in their heyday.
also on a skate vein, 'the man who sould the world': the steve rocco story is very good. it gives a great incite into the way skating changed in the late 80's/90's.
surfing: yes riding giants is very good, i also like dana brown's 'step into liquid'. it showcases some aspects of surfing not normally seen such as the guy who vowed to surf every single day for 28yrs thinking it was a luner year, and guys in the gulf of texas riding supertanker wakes.
for a classic uk documentary, dennis mitchell's 'quentin crisp' documentary is probaly my favourite. beutifully shot and quentin is a classic sound-bite a second. you can watch it on you tube.