This app was mentioned in 2 comments, with an average of 1.00 upvote
I can take this app outside and walk around. Might depend on the phone. Then you'd need to cut a hole in the cardboard for the camera to work. I haven't seen any apps that support that though.
https://play.google.com/store/apps/details?id=com.malumapplications.vrheightphobia
You could probably start today if you have a modern Android or Iphone. Just get a cardboard headset and download this app
*Disclaimer, I've never used the app. just found it in a search.