The best solution is to get them into their permanent outdoor coop & run, as soon as possible. If you are still building it, you may be able to finish the fenced run first, so the chicks can spend their days in it while you finish the coop.But I wasn't thinking about taking them out on warm days and where to put them when I did that. I don't want them in a box where they can't feel the ground so just not sure what to do......
In the meanwhile, you can bring some of the outdoor things into their brooder-- piece of sod, shovelful of dirt, grass clipped into short pieces, a dandelion plant to pick at, and so forth.
If you have a safe yard, but just need to keep chicks from wandering off, you can use a laundry basket upside down on the lawn, or tape cardboard boxes together to make a "fence." But things like those will not provide any protection from predators, so you will have to decide whether the benefit (chicks outside sooner) is worth the risk (hawk, or dog, or whatever.)