I have 10 chicks of various breeds that I brooded in a kiddie-pool in my basement. The breeds are: RIR, buff orp, EE, Speckled Sussex, Barred Rock and White Rock. I tried to keep the basement temperature rather cool (mid 60's), and used a
brinsea eco-glo heater. At 3 weeks, I removed the
brinsea, because they were no longer using it (except to sit on -- so I put in some perches instead). They began feathering up pretty quickly, so at five weeks (one week ago), I put them out in the unheated coop. The temperature has been around 50 degrees F during the day, and it has gotten below freezing a couple of times during the night. So far, they are doing great, and have been growing and eating up a storm since I put them outside (knock on wood). I am a first time chick owner, but from what I've read (and now, observed), they tolerate the cold really well once they have feathers. So, my goal was to gradually wean them off heat and acclimate them to cooler temps as soon as they seemed able to handle it.
Like your chicks, when mine were in our basement, they did not get much sunlight or exercise. I took them outside, for about an hour each time, just twice before I moved them to their coop. But, they were active while in the brooder (some more than others), and became MORE active as they grew. Flying into each other, trying to get out of the brooder when I changed their bedding and food/water....so I knew they were doing well and were getting ready to take the next step. I wanted to get them out of my basement before it got REALLY cold, so they'd have time to get used to it.
They are 6 weeks now. They fly out of the coop the minute I open the door to the run in the morning (even when it was just 30 degrees a few days ago), and put themselves to bed around dusk. They sleep in a row, close together, on their perches, but never "huddle" or pile up-- I went down to check on them a couple of times during the first really cold night. They looked pretty cozy to me, so I went back to bed.
Have you tried taking your chicks outside during the day? Maybe they are not as active because they don't have as much room to fly around. Or maybe the particular breed you have is just more laid-back? I know some of my chicks are more active than others. I was kind of worried about one for the first few weeks (smaller and sleepier than the others), but she has since totally caught up (at least, activity wise -- she is still the smallest).
Anyway, good luck!!