​These are all reasonable suggestions

13 things you'll probably regret doing in your 20s

More below.

I would caveat number 1, however, I only "hit the gym" in my 20s when I was stuck being less active due to school. Otherwise I was never in the gym, I was skiing most winter/spring weekends, sea kayaking most spring and fall weekends (or sometimes canoeing), waterskiing, etc most summer weekends, and then backpacking, hiking, bike riding, or otherwise being active. During my 20s, the gym was only an alternative during college, and even then I usually swam 3-4 days per week, played squash 3 days per week, and then ran, biked and did other outdoor activities.

By the time I hit 30 work made the entire workout process more difficult, so I started to "hit the gym" more often. I like it just fine, but always preferred to be active, not a gym rat. 

2. Worrying about what other people think.

Huh? Really people waste time doing this? If I respect you, I take great care about what you think, but never so much that it interferes in what I think I must do, whether morally, ethically, or personally. There are millions of people near your age, find a few that respect you, and understand, and accept your moral, ethical and personal positions. Forget those contrary.

3. Parents

By 20 you should have a good working relationship with your parents, regardless of whether they are good, great (like me!), or terrible parents. They have advice to offer, you need to adult up, listen to the advice, and understand that you must make your own way in America. This is not a patriarchal society were the oldest living male relative controls the family. Here family is atomized nuclear, and you will need to find your own way. If you differ from your parents on your goals, or plan, you might need to explain it to them. Patience is an adult virtue. Learn it early, learn it well, practice it, and your life will be far better.

4. Debt

Only use credit cards for purchases you can afford to make today, and real emergency purchases. 

Only take on debt for a home, or for education, but with education you must limit your debt to one half of the REAL median income for the career you are pursuing. So, if you are taking a degree in accounting, and the median income for that degree from your college is $50,000 per year, you must limit your college debt, in total to less than $25,000. No exceptions!

Cars should not be a debt creator for you while in your 20s, and only later if you do not need the money, but the math works out in your favor. So, for example, I bought a car a decade ago. My portfolio was on average earning 12% or so per year, but I could get the Home Equity loan for about 5% per year. It did not pay for me to remove money from my investment accounts to buy the car, when I could borrow for much less than I would be losing. So, I borrowed. 

5. Marriage

You will live to be 90+ you have plenty of time, but remember you want kids while you are reasonably young, not 50s.

6. Dating

​There are two dating modes, 1. I am not ready for anything serious. 2. I am ready for serious. Regardless, every time you date, you need to carefully evaluate the person you are with, their personality, and how the two of you interact.You will find there are only so many base personalities, and aberrations. The sooner you figure all of this out, the sooner you will be ready for serious. There is no perfect match, or soul mate, there are only more and less difficult personalities, and aberrations for you to contend with. You must take full responsibility for ensuring your marital match will work for both of you. If you potential spouse does the same, you have a very good chance. 

Even fun I'm not ready dates should be treated exactly the same as an informational employment interview. Take something home from every date, learn something from every person.

If you apply this to your non-dating personal relationships as well, you will be far better off.

7. Investing

Put away a minimum of 15% of every paycheck into a long term "retirement" account. If you can put another 3-5% into an HSA for future medical care, you probably won't need it for many years, compounding interest, and growth investments are magic ways of making future costs much lower.

You will also need to save for the immediate vicissitudes of life. You are better off limiting your expenditures for a year or few and building a slush fund for vicissitudes early. 

8. Travel

Sure travel, but understand that it conflict with much of the above, and traveling with people who are not life partners, or family will be less memorable then traveling with life partners, and family. 

I lived, by myself, in Mexico for most of a year in my very early 20s, I would suggest doing something like this for everyone. It really changed my life. But then most of the rest of my international travel occurred after I married from my 30s on. This worked well.