Well, the easiest medium to begin with is coloured pencil. Prismas can be expensive but they have good results, their leads are very soft which gives you smooth colour.
Another thing that's quite fun is markers. The really good ones (usually Copics or Prismas) can be very expensive though, about $3 a pop :-/ They have awesome results, though. They blend really well and you don't get those annoying streaks.
If you're feeling adventurous, I'd suggest painting. Watercolours are really fun especially if you're into the cartoony style of shading. That'd be because watercolour is a translucent medium; you lay down layers of colours to get your dark>light tones. They can be kinda difficult to control.
I'm pretty sure you've used acrylics before - they dry quickly which allows you to paint in layers, they don't blend as well...and they're an opaque and rather thick. I didn't really like them. I'm used to being able to follow lineart, and you can't do that with acrylics, really.
Now oil paints are amusing. A few days ago I finished a landscape and I can't tell you how much fun it was. ^____________^ They're really thick and greasy, and you can't control them that well... :-/ They don't dry easily though so you can blend the colours well and touch it up even days later. They're perfect for rather abstract stuff.
As for pastels...I never really liked them. :/
The last choice would be digital colouring. This is fun, no mess, no clean-up, the drawing doesn't get worn and torn...but you need the right software for it, and maybe even a tablet. I'd suggest you try traditional mediums before using this...practice with the traditional stuff makes you better in digital. It can be very versatile, you can get all sorts of effects you couldn't normally accomplish. I really like when artists touch up let's say, a watercolour painting and give it these effects they coloudn't possibly get. It looks marvelous. And if you're scared, there's always the undo button.
And now I'm rambling. ;;>.>
