If you had a chance to speak with cadets and/or midshipmen before they commissioned, what wisdom, advice, and lessons would you offer them?

As a junior Naval Officer, the most important thing I did was to ask the Chief's Mess for a mentor. Listen and accept their constructive advise openly. Secondly, spend your O1 and O2 years learning everything about the profession (your job) and the services. Understand the platforms, capabilities. Don't limit yourself to your community.

Coming from the CPO Mess, you will miss the camaraderie and tight bonds of the Goatlocker. The wardroom does not work in the same way, and I've seen that become a friction point for those commissioned into the officer ranks from the Chief's Mess. Most of your fellow junior officers will not have your experience and will be somewhat intimidated by you; share that experience willingly and those bonds will develop.
Others have posted the key points below, especially 2LT Ford.

First of all, thank you for serving our country! The first day when you check in to the command approach everyone with the highest degree of respect regardless of their rank. Lead by example. For example, paint, clean etc. don’t be afraid to get your uniform dirty. Become an expert in your area and all areas of the command therefore read, read, read and guess what read some more. Become a knowledgable leader. Last, ATTENTION TO DETAIL. Observe, inspect and know your area a responsibility like the back of your hand.

Best of luck!

Some great advice above. I didn't read all of it, so forgive me if this is redundant.Here are a couple pieces of advice I've always tried to instill in the Officer's I've trained (Navy Specific):
1) The Chief's Mess is unique and extremely valuable for their expertise, experience and knowledge. As a former member you should know that well, however, as an Officer, it is your name that signs official records. "Trust, but verify" has been mentioned above. In our constant barrage of work I often see people (across the CoC) that just sign documentation as it is completed. My personal pet peeve is evaluations that I have been told "The Mess already took care of it." NO, They have a valuable input, but it is your signature at the bottom of the page. You need to make sure you either agree with what is written or at a minimum have a good explanation of why it came out the way it did.
2) Make a decision and don't be afraid to fail. I know this is particularly difficult in today's military with a "zero default" mentality (especially when promotions are concerned). BUT, I'd rather have someone make a mistake and learn from it than someone who runs to the boss with every decision. If you need advice, ask for it (up or down the CoC) but take that onboard, balance the consequences, and own it. Be deliberate, but wasting time and energy, or worse delaying a difficult decision, is poor leadership.
3) Keep a journal. Assuming you will be "in" for a while yet, it will amaze and reassure you to revisit how you evolve in your decision making process as you gain experience.

Good luck. GO do great things!

1) Stay humble. Your rank is on your collar so no man could look you in the eye and not know you are an officer.
2) Respect your enlisted men. Trust that they can perform until they prove otherwise.
3) Don't embarrass yourself by thinking you know as much as they do. Give them the opportunity to recommend, then you judge their recommendation as to its validity from your level.
4) Remember GOD is watching how you handle responsibility.

Prepare to fail, maybe even a few times before you get it. I tell my Sailors to use resources at the right level and time; there is little excuse for failing alone. Here is a simple wordpress page that I send to enlisted wanting to become officer or others who recently commissioned: https://jorules.wordpress.com/.

Take the time to mold non-prior junior officers. They feel above the enlisted and it shows... making them arrogant and unapproachable.

For yourself, work your butt of to earn your Warfare device as quickly as possible before taking on collateral. Collaterals with slow your progress; and the collaterals they give unqualified officers are low impact. Race to qualify so you can take high visibility and high impact collaterals, which will fast track your progression amongst your peers. That progression will be critical to earning your board for promotion and securing tenure for retirement.

Remember your roots as a prior enlisted. Stay humble.

Enlist first with 4 yes and promotion to e5 with two ncoers first

There is so much to say and it depends greatly on what designator community you are joining. I’ll start by reiterating items that have been said already. Take care of your Sailors and they’ll take care of you. Don’t be a yes man, but if you can learn to pick your battles it can be helpful. I have not and I die on many hills. But take care of your Sailors. Find ways to use the actual electronic means of doing business without printing out copies to put in to a blue folder to be routed that will take weeks for an approval of something that should take no more than two or three days. Take care of your Sailors. Don’t accept the words,”that’s how we have always done it”. You should be pissed off if you hear those words. Take care of your Sailors. The Navy without Sailors is just a bunch of junk that does nothing. Yes, we are a fighting force and yes we need to go to sea; but would you rather be short handed for a week so a Sailor can take care of a legitimate personal issue or risk that Sailor making a mistake that injures or kills themselves and/or others because they are preoccupied with that personal issue? There are some that take extreme advantage of the system and you have to figure out who they are. Bottom line: Take care of your Sailors. That being said, become an expert in your chosen warfare profession as well.