First, congrats on the promotion.
A few things I’ve done:
You might want to think about having an offset day to got look at architecture and furniture, get their brains thinking differently.
Another idea would be to give a short seminar, once per week, on a different topic. Could be about a technique, could be about a particular designer, or analysis of a particular design.
Have an outside presenter come in to do a workshop on a particular skill you want to strengthen. Sketching, paper models…
Set up a regular group crib to review everyone’s work as a team. I do this every two weeks. Obviously we check in frequently on a project basis, but having a crib every two weeks as a team raises the level of expectation on everyone.