Unlock Y the Alien
Unlocking your old friend Y takes some doing - you have to collect all 100 of the alien artefacts in order to do so!
These are spread across all five levels (20 per level) and are hidden inside scenery. This can be anything from a building, to a rock, to a fence, to a meteor, you'll have to scour the landscape in order to find them all.
The easiest way to do this is to put on Free Wheelin' mode and take your time destryong all the scenery.
Alternatively, keep an eye on Project V8 for a forthcoming list of locations...