In most cases there should be a demo script in each one to run a demo.
To make HPA check for metrics faster you should set the
horizontal-pod-autoscaler-sync-period flag to something smaller than the
default (15s). It makes for a much better demo.
To do this in k3d you can do something like:
k3d cluster create rabbit --k3s-arg '--kube-controller-manager-arg=horizontal-pod-autoscaler-sync-period=1s@server:*'
Not all managed K8s providers give you access to this flag.