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In short, I grew up believing I was an INTP and pursued those directions very much. It's kind of sad because I was a huge sports guy (although not the way people think of sports - solo sports like skiing, swimming... and limited sports like tennis) and chose computers as my escape. Although I love technology, I hit a brick wall with technology... I then tried philosophy and psychology with similar brick walls. Eventually I went into finance, although I needed a job in the meantime... and that job led into this one. I'm only now accepting my nature better and starting to adjust the way I do things. Least work is paying for my courses this time.
But honestly, looking back, there are just too many factors with ISTPs to nail it down. Look into finance if there is a math inclination... I was failing math in elementary school, but my parents put me into Kumon - math tutoring thing and now... well, I'm decidedly not bad at it, so don't judge it by school measurements alone. They bought me a computer and I loved that. They put me into chess club, skiing, tennis... just about everything. It was the best thing possible for me. It seems like we come into our own at a later age if we are able to try enough things. The emphasis on past experiences is very important. Most of the stories here are dramatic versions of "wow, I found what I wanted!"... but really, we want to do everything. At some point something from our past or present opens up some kind of door and the rest is history.
Course, this refers to the period between one and two decades ago so things have changed. But I know more recent STPs that loved building models... everything from just models to racing cars to planes/helis. The same SPs show remarkable talent with strange things, like 3d modeling and such.
And not to be unPC yet again today, intelligence plays a large part in this as well. In general, significantly above average SPs seem to have a knack for doing just about everything, but in particular, they tend to be information processors (ie: analysts and such). I thought of that as an INTP trait for a long time until I came across some research on how higher corporate SPs act - seems that's the trend.
So, IMO, it's hard to give any direct advice. Pushing them to do things is probably the single most important thing. Instead of a summer job as McDonalds, a job packing computers (me!) is better... so is volunteer work, so are camps... anything, really. It doesn't even have to depend on means, really, since a lot of the things can be cheap or free.
But motivation might be tough - the sensitive parts for ISTPs is being able to do something. Always emphasise what they'd be able to do (ie: work for a mechanic? Fix a girl's car! Dance? Think Bond!) with their experience.