I would suggest looking at a lot of peoples garages or build threads to get ideas of what you like. I'm not saying to copy others, just that you might like one thing from a few and something else from another. Basically look around for a while and see what you like and don't like.
There are tons of ideas out there, others that haven't been mentioned are custom bedside stickers (unless you like Hello Kitty or the Zombie stuff), aftermarket seat covers, painting your engine cover, window visors, and so on. The list is nearly endless.
First you gotta answer a few questions for yourself.
1) What to you want the truck to do? That will give you a direction to go with your modifications.
2) How much money do you have to spend? I never go cheap...when I can spend more and get quality.
3) Are looks or performance more important to you? I like a mix of the two.
Like TacoDawg mentioned. Spend a lot of time looking at other peoples build threads. That has helped me create a vision for my truck...and put me on to a lot of good companies that supply quality aftermarket parts for our trucks.
Have fun...and don't go broke. The mods can be endless.