t-ob
On the rational numbers, at least, the p-adic metrics are more or less your whole lot, according to Ostrowski's Theorem [1].
There is a kind of cognitive hurdle everyone who studies these numbers has to clear, in that things that should be "large" turn out to be very small indeed, when viewed under a p-adic lens. I think it's more instructive to build up the ring of p-adic integers first [2, chapter 2], and construct the p-adic numbers from there. I can assure you they are very useful, though! A general theme in number theory is to take a "global" problem, defined over the integers, and to translate it into infinitely many "local" ones (over the p-adics, for each prime p). These are sometimes easier to solve and, if you're lucky, offer insight into the global solution you're looking for.
It's terse but comprehensive, and the exercises have been a pleasant source of a-ha moments. There's a more recent version of the course offered, but I haven't been through it.
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I'm no longer in academia and haven't used Sage for years but it's great to see how far this project has come. I hope it gets the funding and development it needs.
A most peculiar series
There are of course many rings in which all prime ideals are maximal. The integers, for example, and more generally the integer ring of any number field (which are not always principal ideal domains, but are examples of Dedekind domains).