When you are used to living on a low income, it is very easy to skip regular doctor visits for eye exams, dental, vaccines, etc. It's also tempting to buy cheap food that happens to be unhealthy. The most valuable thing to you is actually your health, but often times we don't learn this until we lose it. Being young makes you especially blind to it if you aren't already suffering from a disease.
So get used to spending a certain amount of money each year on regular checkups, even if you don't feel like you really need them. It's always easier to prevent a condition from getting worse than to fix something that has progressed to an advanced stage that you wrote off as insignificant.
Food especially is an investment in your health that you make every day of your life. Going cheap on clothes is one thing, but going cheap on food is penny wise and pound foolish. You don't need to buy the most expensive food, but you should try to buy the highest quality, healthiest food you can reasonably afford, and form good eating habits right now. That means reducing frozen/packaged food (which tends to have a lot of sodium) and buying more fresh fruits and vegetables (and eating them rather than throwing them away after a week or two!).
Also, you should try to get 150 min/week of Zone 4 exercise (heart rate at 80-90% of maximum). Whether you have a favorite sport/activity that achieves this, or you need to get a gym membership is up to you. Do the thing that you are most likely to continue for a long time. If you like bike riding, get yourself a nicer bike to motivate yourself to ride it more. If you like to skate, splurge a bit on skates, etc. Find an activity that gets that HR up and splurge a little to help motivate you to stick with it and do it regularly. As an engineer, I am certain that you do more sitting than is healthy and probably not enough activity right now. If you've never tried rock climbing, and you have a gym near you, that is a nice splurge that is both fun and doesn't require much investment.
It's also not a bad idea to get a fitness tracker that helps keep you accountable and can even recruit friends to keep you engaged.
If you are willing to cook your own food, investing in some nice kitchen tools is a good way to spend a bit of money. A quality chef's knife, cutting board, and saute pan go a long way. Having good tools will make it easier to cook and help keep you motivated. If you work from home and like coffee, getting a nice coffee machine can be a good splurge. If you like espresso drinks, there is probably no better investment than a decent equality espresso machine. Of course, you will want to buy whole beans and a grinder to complement that.
If you have a cheap bed, consider investing in a nice memory foam mattress, and possibly upgrading the size. You spend a third of your life in bed, so this is a stupid thing to skimp on. Also get pillows that help give you a restful sleep, even if you have to spend a little extra.
Finally, get yourself a bidet toilet seat. I recommend Toto, especially a model with a water heater. This will change your life, I swear. You will feel like a Neanderthal every time you have to dry wipe, and you will realize how backwards cultures are that have not recognized the obvious benefits of bidets. Again, don't cheap out and get a $50 seat. Those have cold water and are probably not much better than none at all. Look to spend at least $200, but more if you are willing. It should last for many years, and you will probably use it every day. You can fit it to most normal toilets, so it will work even in apartments. It requires no special tools to install.
Many of these suggestions are one-time investments, while others are an ongoing expense. You have to decide how much to spend on them. But at least think about them and weigh them against other possible uses of your hard-earned cash. When you are young, it is easy to take your health for granted. But as you get older, you will be very happy if you spent a little extra time and resources maintaining your health at a high level. This is especially true of sedentary office jobs. Good luck!