I remember that when I was a sophomore in high school I wanted to give a shot at higher maths and started reading some articles there. I found [0], which states that between any pair of distinct real numbers there exists a rational number. It was one of the first proofs I read, and found it so ingenious that I wrote it down and read it while commuting to school. Then I went on to study maths and found that the arguments and reasoning behind it were not that 'marvelous' [1], in fact, mathematics has several results/generalizations with the same underlying ideas (well-ordering principle, unboundedness of the naturals in this case) present as variations in any proof, not just this one. When mathematicians have a new tool we start applying it to everything we can.

Another thing, there is [2] which has not the same purpose: it covers the majority of undergrad math with lots of examples and proofs, it's more like a textbook.

[0] https://proofwiki.org/wiki/Between_two_Real_Numbers_exists_R...

[1] There is some retrospective explaining, of course, the proof was marvelous in ancient times, it's just that in the modern formal abstract setting the ground was laid to characterize number fields with the properties the ancients used.

[2] http://mathonline.wikidot.com/

EDIT: there it says the proof was featured in 2013, the same year I was a sophomore.