We installed a small doweling rod after a few days that was only a few inches off the ground. We let the babies figure it out by themselves... a few never got it, but the ones that did now LOVE roosting, but still won't do it at night (they're a month now). After a few weeks of the small rod, we changed to a thicker one that fit their growing feet better. Just make sure it's secure or it'll roll them off, I learned my lesson on that one!
Some people put a little perch in the brooder. However, in my experience, most don't want to roost for several weeks and I let them decide. I've had birds go 10 weeks or more before they started. Sometimes they'll do it after 2. It's usually after they no longer huddle at night.
My first brooder was a large wire dog crate, lined with cardboard on the inside to prevent drafts. Since I had lined the floor with paper towels, I had placed a few segments of 2"X4"s, anywhere from 6"-12" long to help hold down the paper towels. I noticed right away that some of the chicks seemed to like to "roost" up on the 2"X4"s, especially those that were closer to the heat lamp. When I tuned a segment of 2"X4" on its side, and braced it with another one to keep it from falling over, I always had chicks up on top.
As they got a bit older (a few weeks maybe), I poked two holes into the cardboard on opposite sides of the crate, maybe 5 or 6 inches off the floor and poked a broom handle through the holes. Very quickly the chicks were using the roosts at night. During the final weeks before they were moved outside to the coop I put in a second roost that was about a foot off the floor and that became the favorite.
I just received 35 three day old "Freedom Rangers" on Friday, and started putting the 2"X4" segments on the bottom of their much larger brooder and am seeing that the wood generally has one or two chicks perched on top. So, I guess they enjoy some elevation or change in scenery right out of the shipping crate.
I was amazed at how curious and exploratory my first batch of chicks were. I tried to change up their brooder every few days...a split log from the firewood stack, a tray full of dirt and gravel, a large rock, all were met by a big exploration and interest.
This new bunch are Freedom Rangers, which I'm raising for meat, so I'm trying not to get too attached or spend too much time watching them play, because I'm afraid I won't be able to do the deed...and I really don't need 36 additional chickens...especially since half will be roosters.