Always devote a little bit of time to the things you'll always use. L-canceling, Aerial Interrupts onto platforms and from ledge, MCs, Jab Canceling, pivots, wavelands, wavedashing, different kinds of bomb jump, grapple wall jumps, sweetspots with both grapple and Screw Attack. There may be other things I'm not thinking of, but practicing these things is important to maintaining consistency. Then, you can think about match-up specific stuff. Use random DI and teching to practice reactions and follow-ups, as well as trap execution.
On your own, execution and reactions are the most important things to practice. It's not really possible to practice actual gameplay on your own. Even the best coded AIs don't really hold up to a person that's actively trying to figure you out. If you can get a regular practice partner, I recommend doing so.