While playing against other people is difficult to replace, there are some things you can practice solo.
Start with movement. Simply spend time dashdancing around the stage, making sure that you can adjust the length on the fly as necessary. When you can dashdance from one side of the stage to the other without screwing up, you're probably good.
The next thing you want to do is practice wavedashes. In order to check that your wavedash is frame perfect, roll to the ledge and try to wavedash onto it. If you make it to the ledge, that was a good wavedash, a jump means you're airdodging too early, and a SD means you're airdodging too late. And if you fall off the stage but NOT in freefall, that means you were holding down longer than you need to. (As a side note, don't bother trying to practice this particular trick on Yoshi's, the stage slant means it's impossible with a normal angle.)
It's also good to practice the dashdance and wavedash in combination. Practice getting onto the ledge as fast as possible from various points on stage. Practice using your wavedash to reset the dashdance, or alternating dashes and wavedashes to move across the stage as quickly as possible. Make sure that your wavelands are good, and take care to practice it on different stages (The timing on Yoshi's, battlefield, FoD, and Dreamland are all different.)
Also, make sure you know the spacings where you can down tilt out of your dash and the timing on jump canceling grabs if that's not mastered already. While we're talking about basic stuff, make sure your L-cancels are clean.
If you're looking for more technical things, you can also practice shield dropping and pivoting. They're difficult and rather situational, but also extremely useful when they do come up. (For instance, having a 50-50 kill setup on Puff out of a throw is amazing.) And as long as I'm talking about PewPewUniversity stuff, there's also shield stops and no impact landings you can work on. (Look up his videos on youtube if you don't know what I'm talking about, he explains it better than I can.)
There's also edgeplay you can work on. Practice the hax dash ledge stall; even if you can't get it frame perfect (which makes it invincible), it's still an important mixup. Make sure you can fair at the ledge and regrab, fair/nair as far onstage as possible, waveland right on the edge, or wavedash as far forwards as possible to run past the opponent.
After that, I'd consider practicing combos on a level 1 or level 4 CPU, as they have different DI's. Do NOT practice against level 9's, they just give you bad habits. You can use this to get a feel for some of Marth's kill setups, like how at various percents up tilt can lead into dair, tipper forwards smash or a ken combo if they DI behind you. I'd also recommend you practice platform chasing with up tilt and up airs, though keep in mind that the AI doesn't tech, so you want to hit them when they bounce on the platform, not when they do their getup after missing the tech. On that note, make sure you know the spacing to tipper forwards smash through platforms on yoshi's, battlefield, and stadium.
As I think someone mentioned above, you can practice the chaingrab on CPU's. In particular, you want to get used to doing the dashdance regrab at 21% on no-DI, as that's the hardest part to perform consistently.
Don't bother trying to edgeguard CPU's, they don't recover at all like a human player would, so it's not useful practice.
A couple other things you can do are learning how to short hop double fair, and how to waveland (on the main stage, not platforms) out of an empty jump. There's also run off fastfall forwards air, which is a pretty useful edgeguarding trick in some matchups (though if you miss it, they grab the ledge and you die.)
Finally, if you have the 20XX hackpack, you can use that to practice tech chasing out of forwards throw and down throw.
That's just about everything I can think of for solo practicing. Most of it really comes down to making your movement as smooth as possible in all situations, though I do recommend practicing combos on occasion as well.
