Not knowing where you're at in some things makes this hard.
I can tell you the things a fully-rated person goes out to practice are often, "whatever's in the PTS that I'm usually bad at".
Starting with a few things you're GOOD at first, is a good confidence builder, though. I love.... love love love... absolutely love... steep turns. They're almost never a problem for me. I also love.... love love love... full flap short-field landings.
So I do one or the other (depending on practice area or pattern work) and get to "feeling in the groove" (it is supposed to be FUN after all) and then go do some stuff I don't like as much. (Obviously I can usually fly all of them to PTS standards, but as Dr. Bruce says... Don't Accept Minimum Standards!)
I might go purposefully find a runway that's not aligned with the wind and do some x-wind landings, or maybe break out the brain cells and calculate a problem like "how far can I glide from here" in-flight. (I know Kent plays that particular game too, since he's always figuring out how to glide over large bodies of water.
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Example... If the PTS says I gotta hold altitude +-100 feet (just making this up here), I'll pick a calm or mildly bumpy day and try to hold +-20 feet for the entire flight. Or I'll try to "paint the airspeed needle on" and hold a perfect airspeed for each leg of downwind, base, and final for my aircraft +-1 knot. Something that'll make me work hard for it.
So that kinda led into another option... tighten up the PTS standards if you feel up to it. Cut 'em in half and give yourself a workout, but then also keep in mind whether you stayed within the PTS standards too, so you can pat yourself on the back later if you didn't hit your personal standards... if you're the type that'll "beat yourself up"... I am.
As one of my CFI's said once, "If you can fly it level 50' high, you can fly it level right on your altitude, can't you?" Dang evil CFI's... it was all trimmed and perfect and... yeah, he's right... sigh!
"The standards tighten up for higher ratings anyway, so might as well get used to 'em now!", was another good CFI line.
Later when you can fly with other pilots, they can help with "surprise" things (as can a CFI of course!), like... "So where you going to put this thing down if the engine were to quit right here?"
I don't let other pilots mess with the throttle for those "surprises" like that, but a CFI will certainly pull the engine to idle and make it a real issue, so I usually reach over and do it myself at that point... plus it's a good joke, "I dunno, let's find out."
(You don't want to ever really mean that "I dunno" part, though!)
Other pilots are also great for things like, "If you can't land it on the third touchdown zone marker, you owe me a Coke/Beer/whatever." Awww, man... ok, you're on! Wanna go double or nothing?
Executive summary: You probably know some of your strengths and weaknesses at this point, maybe not all, but challenge yourself. It's a fun way to keep your head in the game.
Another good suggestion here is to pick up any of Barry Schiff's "The Proficient Pilot" series of books -- he has good stuff in there that pilots can and should go do, like flying a pattern and figuring out, down to the knot, your particular airplane's cruise speed... or finding out by actively writing down a combination of power settings and flap settings that result in specific desired airspeeds starting from a high altitude over the practice area (you can do these on a long XC too...)... so you can make up a little cheat sheet for your airplane and memorize it.
All sorts of good stuff in those books. Maybe targeted a little more for a pilot who's past the rating and wants to learn a bit more, but certainly nothing wrong with reading them, and getting a leg up on being a well-rounded aviator!
There's all sorts of fun ways to challenge the brain, and always something still to be learned about aviating. It's just that it's been a long time, and I don't remember what I knew and didn't know to recommend something at the "almost done with all the solo time and had the long XC done" stage.
I seem to recall that my instructor said something similar to yours... at this point, your flying is pretty decent, now you gotta hit the books and do a lot of oral quizzes to get the knowledge flowing freely from your brain and mouth for the checkride later. You'll keep flying, but crackin' the books and doing the oral stuff is real important at some stage that I've long-forgotten.
Then you find yourself flying again, reciting the PTS standards out loud from memory and the instructor quizzing and drilling you with oral questions from the right seat while you're doing Private PTS maneuvers at the same time. Brain overload, great "distractions".
You get a feel for what real distractions can do to the flying you thought you had "down cold" at that point in the training and learn to tell people in the right seat to wait, be quiet, or otherwise stop overloading you as you prioritize as an almost-ready-to-be-PIC.
Not too long after that the CFI will do a good review of your logbook, and start the process to schedule a DE... make sure you're prepped and have your paperwork in order, and before you know it, you'll be sitting in an airplane on the ramp with a stranger who's quietly watching your every move, and you'll be thinking, "Am I ready for this?! Really?!"
It's a great experience, start to finish. Savor it, because you can't really re-live it. It's similar for other ratings I hear, and checkrides are always a bit of a raise of the pulse and blood pressure...
But nothing is ever quite like that first Private checkride and the prep leading up to it. You can't really describe it until you've done it, and you can't really ever go back and do it again, either.
I *still* want to fix one of the crappy landings I made on my Private checkride, over a decade ago. I can still see that one landing going to crap in my mind's eye. I recovered "okay" enough to pass that day, and the other landings were dead on... but that one still is there, in the back of my head, along with the sweat and warm day, adrenaline and nervous knees when I got out of the plane, and walked to the DE's office upstairs of a hangar, and... all of it. I can still see the office and remember the patient quiet questions asked before the flight during the oral, and the little look that said, "Is there any more?" when I forgot something and had to think harder or say I knew exactly where to look it up.
And it wasn't THAT bad a landing either really, but it gives a feel for how that day is forever burnt into your mind.
