I left my Rebel stock for a long time, I really wanted to get a good feel for the bike and see what mods I needed to make. Here is what I have done so far and why:
Drag Bars: I really like the riding position with the drag bars. They sit about 2 inches lower than the stock bars, I get a very slight lean forward and my hands are a bit wider apart. Much more comfortable for me.
Hand Grips: I got these at a swap meet, they are a copy of the Kuryakyn grips. These grips do a great job of eliminating the vibrations in the handlebars.
LED Headlight: Street lights are few and far between in my neck of the woods, also have the added risk of deer and the occasional black bear running across the road so I need all the light I can get!
TBR Exhaust: This was the one thing I did for looks. The fact that it sounds pretty mean was an added bonus!
Bigger footpegs: Wife was complaining of foot pain after a ride, going with the bigger pegs has taken care of that. I ordered black but got chrome pegs, I decided I like them so I kept them.
MOLLE sidebag: I needed a little something just to hold a few items when I ride, this holds all my stuff and then some.
Backrest: Bought this off ebay. My wife kept telling me she was afraid of falling off the back of the bike so I added the backrest.
I've gone back and forth about changing the brick and turn signals, but I think I'm going to leave them alone. They work and have great visibility, so unless I find a really sweet looking setup that I gotta have, the lights are going to stay stock.
Last thing I want to do is swap out the seats for one of those double seats. I'm OK with the stock seat, but my wife hates the passenger seat after about 15-20 minutes. That probably won't happen until early next year, we have a trip coming up and then the holidays so right now there's no money in the budget for cool motorcycle stuff.