I get it. All you haters out there don't act like you, or your buddy, wasn't doing the same thing at 17. Let him experience life and learn the same things we did, the same way.
Option A:
You could probably use a generic coil booster. It's cheap and not permanent. Remember you get what you pay for.
https://www.amazon.com/Superior-18-8001-Twist-Front-Spacer/dp/B00029K21G/ref=sr_1_5?s=automotive&ie=UTF8&qid=1478049608&sr=1-5&keywords=coil+spring+booster
Option B
A generic Air lift. I don't know much about them other than they are generally used to help reduce body roll when towing. I have no idea how much lift you will or won't get with it. It's probably better than the coil booster, but it's also much more expensive.
https://www.realtruck.com/suspension/toyota/highlander/2007/
however...Some stuff to consider.
If you lift it and go with bigger tires it will stress your suspension, and stuff may not work right. Look at all the guys that donk out the Cutlass. They lifted is somehow. In the end they can't turn with the 22" wheels on the thing. They have to a 12 point turn just to park it. You may have similar problems. Make sure stuff doesn't rub or interfere with other parts of your car, tie rods, fenders, exhaust, ... The wheel offset has an affect that as well as the diameter of the wheel.
The larger heavier tires will put stress on your axles. My 32's are about 75lbs each! Compared to a factory wheel at about 35 lbs, that's a lot more loading on the engine, axles, diffs, and transfer case. All these part will wear out quicker.
The ride will be stiffer by adding a spacer in between the coils. You are reducing the suspension travel. This will transfer stress the body and other components attached to it. In the old days we would lower our cars by chopping a coil out. Then we started bending our strut supports, which was part of the body. That's really not repairable. I'm not sure about the highlander suspension, but look at it.
I'm not sure how much you can adjust the alignment on it. So you might end up quickly wearing out the pretty new tires you buy.
If you have any sort of electronics that use the wheel diameter, don't expect them to work correctly.
Don't expect to do much, if any actual off roading. Some flat gravel road would be fine, but there are no skid plates to speak of on it. Also some hard bumps at speed could actually damage the unibody. Remember there is no frame.
Option C (recommended)
If it's only the look you want get some BFG ATs or other aggressive tires. BFG ATs look plenty meaty, not too loud, and very good off road. You can probably get them in a stock size too.
Then when you turn 18, sell the Highlander and buy an older 4runner/ Tacoma. You should be able to get something in decent shape for $10K. Maybe a 4runner would be better. You can sleep in the back, if your parents might kick you out for selling the highlander. :winking:
Best of luck with your adventure.