The leveling mechanic of Psychonauts is a marriage of Exploring and Achieving that is a song to my EASK heart. Reaching the level cap is painful, but you only need level 30 (of 101) to have the requisite abilities. Past that, it’s all ability upgrades for convenience and more power. And how do we do this leveling? We find stuff. Every psychic level has a bunch of figments, every physical zone has psi cards and psi challenge marker, and then a few other miscellaneous collections. The late game lets you add an entire hit point bar from a collection quest.
Most of your levels come from normal play, picking up figments effortlessly plus a lot more to gain with just a little effort. In most levels, the figments will guide you through the level, pointing out a good place to head next and serving as an indicator that you have not gone somewhere yet. Then the maps are decorated with hidden spots, platforming, and the occasional mechanics puzzle. Some items are ridiculously hidden, but most are visible from a distance or have something more visible right next to them, like a series of psi cards in the area of a psi challenge marker.
There is a reward for exploring the map. That’s nice. It is also not required to complete the game, since the first couple of upgrades are nice, but then they are mostly convenience or crap until you start getting awesome abilities past level 70 (damaging levitation, damaging shield, regeneration, infinite ammo).