A lot depends on whether you have someone who can show you how to do things, or if you're figuring it out from the manual (and youtube videos, if you find them useful), and also on your personal ability to deal with frustration. If that's low, start with something easy: a stuff sack as you suggest is a good thing. I recently made a few grocery sacks out of 70d silnylon mostly to see how one of my machines would handle the material, but also because they're a useful thing (50 g for 1/6 bbl sack seams pretty shameful around here, but they do hold 15 pounds or so for the mile walk from the store.) If your tolerance for getting things wrong the first time is higher, you can start with someting more complicated. I wouldn't start with modification, because that can be difficult (sometimes more difficult that you think when you start), and screwing up can ruin a good piece of equipment.
Get some cheap fabric, some hardware, a good seam ripper and start messing around.