Shoot in Av mode - let the cam pick the shutter speed. But be aware of it in case it's too slow for freezing movement. Unless you are delierately trying to limit dof, realize most consumer lenses do better stopped down a bit. There is no shame in f/8 outdoors. If you are shootnig a canon cam, up to iso 400 looks all pretty much the same, but outdoors, you are correct, drop it down to 100 or 200 - it'll give you more leeway for using shallower dof.
I shoot ISO 400 without hesitation at all on the crop cams and ISO 800 on the 5D without a second thought when I need more shutter speed.
for shooting without flash i find manual completely unnecessary. As long as you understand exposure compensation, you dont ordinarily need the heartache of manual.