You know how you can access the Eastern Tower after beating Micren? If you go up to Tower RR, then to the right is a room with terminals (and a shutter with a treasure chest). After Sycor is beaten in the Labyrinth, the terminals are accessible.
(The only way to get to the Reservoir is by beating Sycor, but if you cheat to get around that... Sycor can't be rematched, either. )
Those terminals will be usable, and each terminal sets a flag and unsets several of those. Then, after the cutscene, the song selected is chosen.
After beating Sycor again, the shutter is removed and the treasure can safely be obtained - what is it? Is it a bird? Is it a plane? No, it's a cake!

As for the puzzles... well, if you can just waltz in it'd be pretty dull, in my opinion
(But relax, it's easy.)