Wow, I've been gone from this forum for awhile but just came back to check if there was a section for Eastcoast brides (my cousin is getting married in May)..anyway..
Bars in Cabo...you will not find a lack of them in town that's for sure! If you are looking for something low key (not a dance club) then I would take them to the marina. There are lots of restaurants, bars and shops with seating right out on the boardwalk. If you get sick of one place you can always walk over to another. I'd start with Nowhere Bar or the Tiki Bar.
Otherwise, the place we ALWAYS hang out at (I've been to Cabo now 5 times in 3 years) is just outside of Cabo Wabo. There are three outdoor bars mixed in with the little shops and there is always a fun group of people there. One of them is called the Jungle Bar and one is Slim's the Smallest Bar in the World. The one in the middle of those two is where I suggest you go (sorry I can't think of the name). And if you want to dance/do contests/drink shots with people shaking your head and blowing whistles in your face, Cabo Wabo is right upstairs or the Giggling Marlin is right across the street.
Another relaxing place is Hemmingways (it has a classy atmosphere and cigar bar)
Hope that helps!