Which lens next for my Canon kit?

I am a (very) novice photographer who has just received his first digital SLR, a Canon 450D with 2 lenses, an EFS 18-55mm and a EFS 55-250mm. I am experimenting and getting the hang of it and turning out some nice shots. Which lens should I add to my kit or should I go for some other piece of equipment like a speedlite or tripod. Thanks all.