I’m not sure these will qualify as “out of the box”, but here are a few suggestions (obviously you know your group better than I to determine appropriateness):
-continuous forward lunge step up (every step or every other step)
-continuous or static forward step up with hip extension and/or hamstring curl
-continuous forward run ups (skipping every two or three steps)
-continuous or static lateral single step up and/or shuffle
-continuous or static lateral single steps up with hip abduction and/or knee up
-forward bear crawl (not sure if this would be comfortable…)
Your knowledge and imagination will provide you with so many more!