My guess is that Hinrichs and his cohorts were going to do to the stadium what McVeigh and his cohorts did to the Murrah building. They planned to use a vehicle (perhaps Hinrichs' car) filled with ANFO triggered by TATP. The fact that Hinrichs tried to obtain ammonium nitrate a couple of days before the bombing indicates he wanted it for that attack. But he failed to obtain the AN, so they went with a back-up plan instead: putting a backpack with the TATP inside the stadium.
I think it's very hard to buy ammonium nitrate these days. I was listening to a gardening show on the radio the other day, and the host was saying that nowadays you have to buy calcium nitrate instead of ammonium nitrate. Perhaps this hasn't been publicized (except by this talk show host) in hopes of nabbing any Jihadists -- like Hinrichs -- who haven't gotten the news yet.