In my .vimrc, I have:
imap <Leader>ip opp "p$a: #{p$a.inspect}"
nmap <leader>ip yiwi<Leader>ip
vmap <leader>ip yi<Leader>ip
Which add a line to pretty-print the var at the cursor position, such as pp "var: #{var.inspect}". Not as fancy as a proper debugger, but it helps.
