It is easier to ask forgiveness than ask permission.
Exactly the kind of advice I'd expect from someone with the word "Shifty" in his/her user name...
But this is risky advice and I caution against it. Some PI's would forgive you, but some would not. Some would be furious that you were effectively getting paid salary to work in their lab, and using some of that time to work for someone else. They see their lab as your
first only priority, you could permanently damage their opinion of you.
So I really would ask permission first. If you refuse that advice, at least do some detective work and see what other students have done and gotten away with.
I've done tutoring and charged $20-$30 for general or organic chemistry. In my experience, this is not as great as it sounds. If the location is inconvenient, you might only be making less than $10 if it takes time to get where the tutee wants to meet you. If you have to pay for parking that eats into your costs. You can set some geographical restrictions, but then you run the risk of losing customers. The worst part for you is that if you really need the money, this would not be a very
reliable stream of income. You get lots of interest right before a test, and then long periods where not many people want your help. I'd suggest keeping this as a back-up plan.
If I were you, I'd try to look for a job teaching a laboratory section for a science class at a local community college (or even university). Surely if you're in an MD/PhD program there must be something you're marginally qualified to teach, right? Chemistry or biology? A lot of biology departments have trouble finding people qualified to teach anatomy classes, but with a med school background you should be a good fit. It can be hard to get your foot in the door with these jobs, but once you're in you're in. If you can get one section that should be about 5 hours of work a week including prep work and grading, and you might make somewhere from $50 to $100 for your time. If you do well, they might give you two sections the next quarter, which makes your prep work more efficient.
Another idea... you could try picking up a part-time job at a restaurant. You don't make a ton of money, but it's
steady income, no prep work other than putting on the right clothes and showing up, leave the worries at the door, lots of places will give you time off when you need it, and with tips you might actually make more than tutoring or other ideas.
The computer idea... That sounds like a fishing expedition. I don't know where you live, but in my city (Portland) you can't throw a rock without hitting a freelance web designer who has a portfolio and everything. The market is saturated.
But hey, think creatively! You can sell your plasma! Maybe you could become a sperm donor! You need a crazy-high sperm count for this, but if you're already near a hospital... and if you're already in the mood
then it's easy money!