Walking. Just walking.
I realize that doesn't sound very extreme, so here's what I do to turn it into an extreme sport...
I grab an ipod filled with praise music, and I worship and pray while I walk.
I think prayer must shake the foundations of hell.
You can't get much more extreme than that.
“Pray often; for prayer is a shield to the soul, a sacrifice to God and a scourge for Satan.” John Bunyan