Once you're sufficiently good at Nethack to ascend as a Monk or Tourist, you might like to try some form of restricted ascension. This means that you try to win while playing under some extra constraints to make the game harder. For example, you might try to ascend without using any wishes, or you might play as a pacifist (never kill anything yourself). Some restrictions are tracked by the game - type #conduct at the beginning of a new game of Nethack to see them - and you can also make up your own restrictions.
The first restricted ascension I achieved was based on the philosophy of taking what the game throws at you, rather than forcing it to give you what you want. That meant letting the computer pick my character, and no wishing, genociding, polypiling, engraving Elbereth, consorting with incubi or succubi, wraith-luring or nurse-dancing. Not being able to wish was quite annoying, but it was doing without genocide that was hardest, because of the hordes of powerful liches and mind flayers. In the end I turned them all to stone with cockatrice eggs.
Other restricted ascensions I've done are an illiterate Barbarian, a Wizard who never hit with a wielded weapon, a Monk who behaved like a Monk (e.g., no eating corpses or fighting with wielded weapons), and a Healer who took her original pet all the way to heaven with her.