I've tried several fillers, many of which are listed in this thread. Here are my personal picks...

For balsa, the super light weight vinyl wall spackle gets my thumbs up. It is cheap, unbelievably light, applies easy, dries to where it can be sanded in under 1 hour, and sands as easy as balsa and covering sticks to it just fine. The only down side I can think of is that it is white, not balsa colored, but that has never bothered me. You can pick the stuff up at nearly any hardware type store. This stuff is not structural, but fillers shouldn't be structural anyway.

If you must fill a gap in a structural joint, balsa dust (or baking soda) and CA seems to work OK, but I prefer epoxy as it is less brittle and easier to apply/work. Typically I would mix the epoxy with microfibers (fine cut cotton, different than milled glass and balloons) if the gap is large. The microfibers lighten the mix somewhat and add a lot of strenght. However, none of these mixes (CA or Epoxy) sand well, but we are taking structural stuff here so sanding is a secondary consideration.

For glass, other composites, I've tried a lot of stuff as fillers. Epoxy/balloons works well, but once I tried Evercoat (brand/product name) I pretty much use it exclusively. It is a polystyrene product, similar to Bondo, but flows like warm honey. Very easy to work, sands easy, bonds well to composite parts and typically a leaves smooth, non-pitted surface after sanding. And unlike epoxy, the Evercoat can be sanded in about 15 minutes. The stuff is moderately heavy, but again, fillers should only be for cosmetic touch up, not structure, so the weight isn't a big deal since you should be using a very small quantity. The main downside of Evercoat is that being a polystyrene, it stinks really bad and it can be hard to find. It is generally only sold at professional auto body paint stores.

Cheers