You'll end up back here after you defeat the brain and Sasha tells you that he has to leave on some urgent Psychonaut business. Cruller pops out of your ear to talk with you a bit and share some ideas, off to the boathouse to meet up with Lily!
Right when you get here a scene will automatically take place. Well, that leaves quite a task at our hands. Go out on the boardwalk over where cotton candy head used to be, but he's not any more so we can get to this crazy thing we need to drive, but first run straight across the walk without making any turns and you'll get a Psi-Card (you've probably seen/tried to get this card before). You can also get up on the big rock right next to you here and then up on the roof of the area right next to you as well where another Psi-Card sits.
From here look back over the boardwalk you just came from and you should be able to see that there are some more Psi-Cards out there. Use your levitation ball to get a good jump and float over to the roofs that cover the boardwalk. You'll pick up 1 Psi-Card on the second to last one, then jump on the next one and you can walk on the rope up to another, continue all the way up this rope and you'll get on some netting you can climb up to get to a little wood plank with another Psi-Card.
There is a rope going off the back of this area we can walk down and then jump to a platform on our left, then up on a higher platform slightly to the left, and then more so to the left is another Psi-Card just a jump away. Now, beware to your right is a cougar type animal in a little cave, if you get too close to this animal it will try to light you on fire! About 4 shots should take out the cougar.
Now get back on the boardwalk and go over to the thing Cruller told you about; press 'Y' to open it and walk in.