There is a half buried chest in the maze that can not be opened (no key, and Lockpick doesn't work).
There are two hidden rooms, so you need to find the second lever in order to get out of the maze. Check the map to see where the other room should be, or just look more carefully (there is one pressure plate on a wall beside a chest, possibly the one you already found,... and I don't recall where the other one is).
Summoned creatures may not give any experience points. You likely got some for the first set in each room (though possibly not if you were too high a level above them), but after that they stop giving experience points.
There are a couple places that can spawn opponents repeatedly, which will keep giving experience (based on the level difference).
In the mirror room test in Maxos temple, there is an empty barrel by the wall opposite of where you find the solution to the test, which spawns opponents each time it is opened. Later in the game, in Keara's flying fortress if you try to open the door to the inner headquarters without correctly solving a statue puzzle, that also spawns opponents. The spawned opponents in Keara's fortress have a minimum experience reward (doesn't drop to zero), so you can level there a fair bit if you don't mind the grind. That is late in the D2:ED section of the game, though, and there are places in FoV where opponents automatically respawn which give much more experience, so there is little point in power levelling before then.
