Focus has a gym and is reasonable, Andles has a leisure centre, as does the Ambassador I believe. Ibis, Campanile, Novotel are across the road from a large gym.
i recomend the latest hotel that was built in Lodz:
doubletree3.hilton.com/en/hotels/poland/doubletree-by-hilt on-hotel-lodz-LCJDLDI/index.html
its very close to the "downtown", it takes less than few minutes of walk to get there or just 1 bus stop
Best regards :)