A lot of devtools and configuration oddities can be learnt in an hour or less. One can become a fairly proficient/well informed user of any of these. The $VALUE for any/all of these are huge !

  * keyboard shortcuts for your preferred UIs
  * vim (/ your preferred $EDITOR)
  * tmux
  * make
  * readline
  * strace
  * gdb
  * valrind
  * tcpdump
  * wireshark
  * basic bash
  * all sort of server configs:
     - nginx
     - apache
     - postfix
     - ldap
     -...
  * compiling the linux kernel
  * understanding the sysctl tunables
  * pick a binary file format and understand it
  * pick a protocol and understand it
  * man proc 
  * iptables
  ...
  ...

You get the general idea. Personally, I sometimes feel the only reason I am respected by my colleagues is simply because I know a little about a lot.

If you have an hour to spend, don't look for the biggest bang of the buck / the most valuable. Explore something small that you use all the time / wish to understand better but didn't take the time.