One technique i used to good effect in the old analog days involved a graphic EQ. Split the mono signal into two tracks, pan one hard left, the other hard right. Add 20 band EQ to each track, and set the left track to have alternate bands high and low. Set the right track to the opposite pattern. As long as you don't over do it, it will add some separation to the channels.

What you were doing with the time offset is to effectively add a very short echo. This isn't as noticeable through stereo speakers because your mind is used to correcting for time delay differences between the ears (when was the last time you listened to an entire album while sitting still precisely in the middle between the two speakers?) and does it so automatically that you don't even notice. When played through a mono system, the brain loses the spatial cues it uses to make this correction.