I am, as always a person who suggests Railcraft. For this pack in particular it adds several useful things. First it's a slightly better introduction to Multi-block construction than Forestry. It adds another method of generating steam for power, as well as another way to use both Buildcraft Fuel and Forestry Ethanol. Plus that same method explodes when it's done wrong, which is a valuable learning experience, as well as amusing when it happens. (To someone else.) It also expands on Buildcraft's ability to transport items and fluids, without stepping on Buildcraft's toes. And since Tesseracts are pretty expensive, you can get a Railcraft line up and running to power a quarry without needing to hunt down two stacks of ender pearls before you get to it. Plus there's a long way to go if you decide to go deep into the mod. Signalling and Routing give some useful depth to the mod, but aren't necessary so no one gets confused before they're ready to make the stuff. Plus the danger of getting yourself run over is ever present and the death messages are amusing enough to keep the mood light when you do inevitably get splatted. Also only adds ores already present, though you'd probably want to disable Poor Ore generation in the config.
Alternately, Storage Drawers. Incredibly simple to understand, and versatile. Plus it looks very cool. A simple storage expansion option that should be light on the system. (Doesn't add any new ores.)