There is a lot stories describing the origin of the name - Simple Plan:
One of them is, that once the guys watched the movie "A Simple Plan" and they decided it's a cool name for a band.
Another is, the the guys claim, that they never wanted to be stucked in some office, but they wanted to play in a band and discover the world. It was some kind of a simple plan for them. And that's why they are called so.
According to one recent interview with David, it happened this way:
"The guys were like “we need a band name, we’re playing a show!” and the producer guy had just been watching the movie ‘A Simple Plan’, and he was like “you guys should try that name”, so they dropped the ‘A’ and just went with Simple Plan, and we ended up keeping the name. When we got signed we were wondering if we were keeping that name or not, and you know what, it is kinda Simple Plan because we get to do what we’ve always wanted to do for a living."
Little curiosity: While thinking up the name of the band, Mark Hoppus (Blink 182) came and suggested a name "one legged duck". However this name was rejected.