Find a string of pubs/bars that are within walking distance of each other. Verify they all have a similar time-frame for Happy Hour. Map out your route-to-take; park your car at one place, walk to the further bar, and work your way back.
(or call a cab/Uber/Lyft)
Sometimes Happy Hour is whenever you say it is. I mean really, if you think about it, it's basically a state of mind (that also includes drink and/or food specials which happened to be based outside of your mind and within reality).
But all that aside,
You can string together a series of pubs on Google Street View or even use something like MapMyRun to create a "rum route"