Tank Bag

A good tank bag has a magnet in the middle and two or four 'arms' with magnets on them. It stays very solid on the bike, and also has a clear vinyl front to place maps and such behind (convenient for DVD rentals, actually). But you'll want to keep your interac and credit cards away because those strong magnets would kill them almost instantly.

Marsee, Joe Rocket, Tour Master, Nelson-Rigg and many others make excellent tank bags. These are available from very tiny size (big enough for a baseball) all the way to "massive touring" size, which obscures all view of the speedometer and tachometer. The Tourmaster Magnetic bag fits well and holds lots of stuff:

A tank bag is a good choice if you only have a bit of stuff to carry. A reasonably sized tank bag will fit a sweater and rain jacket, or binoculars and a camera, and so on. A carefully packed larger tank bag can hold enough clothing for a weekend trip.