Slight Aside: "Not really, maybe once or twice in an adventure" is an oxymoron for me as I generally have one or two combats ONLY in an adventure.
In general I tend to plan out likely paths for an adventure, and script encounters for them, but my players commonly will then do something different, and I will often use a random encounter for that. Or semi-random.
I've been running Pendragon for the last few years, and I've been using the following system for unscripted encounters:
- I choose a set of potential adversaries based on the play situation -- about a half-dozen to a dozen.
- A player makes a skill roll to see how well they can read the situation (hunting or battle are common)., That determines how many random cards they get and choose one from.
- I often have a set of special encounters for critical rolls.
It works nicely for Pendragon as the fiction supports the action ("I succeed at hunting" -- "you pick up the spoor of a panther and a wyvern, which do you follow ...") and blends player knowledge, GM scene-setting, and character abilities nicely.
You could easily adapt it to a random encounter table -- basic success the players get the choice of 2 encounters, 5+_ over they get three, and critical they get a special.