We have 7 chicks aged 3-4 weeks and 2 weeks ago we felt the same, that they seemed bored... so to stave off any mischievous thoughts, to our brooder we started adding 'toys' and we now have (all found in the yard or around the house, no purchases): roosting bars at 2 different heights, a chicken swing, a little ladder, a mirror, and a Tupperware container full of dirt/sand. Most days the kids and I add clumps of weeds or grass we pull out by the roots, and cut ants or other worms/bugs we find in the yard. Treats in very small quantities go in daily including dried mealworms, cracked corn, yogurt, watermelon. (I started them on chick grit for a week before I introduced any treats)
What they seem to LOVE are the roosting bars (especially the higher one), the sandbox, the leaf cutter ants, and the grass/weeds. This weekend to change it up I tried hanging the grass clumps (roots down) from the metal grate I use as a cover on the brooder box and it's been really entertaining for them (and us!). They hop around trying to get to different parts of the roots. We add the leaf cuter ants to the sandbox and it's hilarious to watch them go after the ants and chase the chick who catches an ant.
What they don't seem to care for much - which surprised me - are the mealworms (maybe b/c they are dried. I tried rehydrating them with some broth but still not much interest) & the swing.
Here are some pics of the toys in our brooder - in the one you can see the whole setup and in the other you can see one of our chicks playing in front of the mirror.
Good luck with your chicks!!