Two of my kids worked from age 16 at an ice cream shop and made some serious money in tips, especially during the summer and holiday seasons. Learned a lot, too. Third child got a very cool paid internship for the last two years of high school. It was a highlight of her college application. Also paid $10 per hour, which was pretty nice back then.
My girls both made good money babysitting. One daughter was even give a car when the family moved out of state.
My son worked for a landscaper during the summer. All my kids did dog sitting for the neighbors.
In many places - none. This is the truth of the current economic conditions when half of able and experienced adults could not find the jobs and stopped looking. “A great summer job is life guarding” - it is infinitely hard, practically impossible, got to have connections (may help and may not!). My competitive swimmer D. who knew most of the lifeguards in our city because of her competitions, could not get one in better economy. If you get it, good for you!!! She worked at Research lab for 2 summers because of her personal connections. But then, after the next hike in min. wage, guess, what happened to her? Yes, you guessed it right - she was let go, they could not afford her anymore.
Last summer my son worked for Boomer’s, one of those places with an arcade, mini golf, laser tag, go karts, etc etc. I just checked and they ask that workers be at least 16. That’s a great seasonal job, he got loads of hours in the summer months, the place is crazy busy! I’m sure most areas have something like that, even if it’s not Boomer’s per se.
D has worked as a camp counselor x 2 summers. She grew up going to this camp at least for 1-2 weeks every summer. She loves everything about camp…the campers, the co-workers, her tan and the camp itself. Unfortunately when you add up what she really makes it comes out to be like 3.00 an hour. And the time commitment is intense: 7:30 am until almost 5. She comes home exhausted. Even though you don’t have to work every week of the summer, it still is more time than she will have this summer.
So she took a job working at a locally owned sandwich shop. So far it is working out great with good hours and good pay. I worry about her being exhausted but so far she says she is good. She does not play any sports and she quit her main EC (orchestra) this past year. Her class rank is high and it is important to her that she keep it. This is the first time she has worked during the school year and it makes me nervous!
There are no “summer/part-time” jobs for college bound students anymore. Now they are called internships even if they are at McDonald’s or the pool.
It really depends on the area and kid. My S was a volunteer to a summer science program after HS JR year, which led to him being hired there for the next 2 summers. My nephews have worked at a stand, renting out beach equipment. Babysitting and tutoring are other jobs that can vary on availability. Teaching computer skills and tech support for a senior center or community might work. My niece hustled and got a county job as a recreation leader which paid better than minimum wage.
Back some decades, jobs I had in HS included babysitting, tutoring, office duping clerk, sales/retail, and hotel switchboard operator. I feel each gave me a new skill set.
When he was 17, my son worked as a pizza and sandwich delivery driver. (Using our car, of course.) He served a college neighborhood and made a fortune in tips. Stoned college guys needing munchies would give him fists of money when he delivered the food. To this day, he laughs at how much fun it was.
My D got her first job using snagajob.com. She worked for a year at Arby’s then got a new job last November at Meineke as a tire changer / oil changer person. She really seems to like it. Good luck!
The problem with under 18 is that some places have restrictions on how late a kid can work so businesses won’t hire them because it makes it too hard to figure out a schedule that has closing hours after 10pm. Check your states rules on the restrictions that your teen can work.
Do you have a tutoring center like a Kumon or a Mathnasium in your area? Both my kids worked at the local Kumon as a grader during junior and senior year of high school. The owner of our local franchise hires high school kids to work a couple 4 hour shifts a week after school, say 3 to 7, to grade homework packets that her students bring in or help kids as they work on their packets at the center. She paid a $11 per hour as I remember, which was good spending/saving money for my highschoolers. And it was a welcome change from fast food service!
My kids all started working the summers they were 15. As the OP can see there are so many options. I’m a proponent of kids working for many reasons. Both of my college grads found real jobs fairly quickly after they graduated in part because they had an actual resume with real jobs spanning 6 years. It can be anything from tutoring to lifeguarding to working in a restaurant or a store…real jobs where you punch a clock and get paid is invaluable and 17 is plenty old to get started. i posted once that one of mine learned a second language during a summer job working with migrant workers and got paid for it and I didn’t have to fund some expensive immersion program. 
Kids don’t learn how to work with other people by osmosis and they don’t learn the value of money until they’ve worked an entire day and find out the money they are paid won’t fill a tank of gas.
Oh, 18 year olds can serve here too. It still surprises me that a restaurant wouldn’t make sure someone was 18 before hiring her fit a job that required serving. Makes more sense if it was someone they knew and it just didn’t occur to them.
Here you have to get a license from the city before you can sell or serve so businesses tend to be pretty on top of that. They will usually ask right up front if you have a license and if not they will explain what you need to do (which would include being 18).