I am very new in my pen turning journey so my experience with this is minimal.
What little I have done is single tube for click models as well as some basic segmenting. One of my favorites (and one which I have had multiple requests for since making) is adding some crushed opal like used in ring turning. Basically turning a very small ring. It uses very little material but adds a distinct pop.
I tend to shy away from anything that adds bulk to the pen, as the entire point of the slimline is to maintain a smaller diameter profile.