When I learned of the new data.tree -> list built-in conversion, I had just had to take a peek with the listviewer htmlwidget (see post). Thanks so much to @gluc for building something I have always wanted.
#devtools::install_github("gluc/data.tree") ### list convert not yet on CRAN
data(acme)
l <- acme$ToList()
library(listviewer)
jsonedit(l)