you need something independent of the scoring.

I would create a sprite class for this.
Create it when the bonus is achieved and add a new instance to the sprite engine.

The TBonusSprite instance would count down a fixed number of ticks (or it could be time based, your call) and then die... possibly even setting it's alpha value on the way so you get a nice fade out. The text would be provided by an image... or you could useTextOut, but you can make it very dramatic by using an image.

You can also make it play a sound ("BONUS!!!!" in a deep echoing voice) and also maybe create a particle effect.. all of this by creating one object when a bonus is received.

Your call, but that's how I would do it.