I spent hours looking for one crystal, so when I finally decided to start again from scratch, I thought I'd write these down for anyone else who might have trouble with it. Many thanks to colinp_uk at xbox360achievements.org for listing in the forums how many collectibles were on each level. That was a lifesaver.
There are a couple of things to keep in mind as you go. First, always keep count of how many crystals/smoothies you have in each level. That way, you'll know which level to re-search if you miss one. Secondly, if you pick up a collectible and die before reaching a checkpoint or save point, you'll lose the collectible, so you'll need to pick it up again. It's difficult to get killed in this game in battle (even in hard mode), but it's pretty easy to go off a ledge, so be aware that you may have lost a collectible this way.
There are other green canisters besides those listed here, but I'm interested only in the collectibles.
CC = Codon Crystal
PS = Powermax Smoothy
Level One - Training Time: 1 CC and 1 PS
1. CC - behind puzzle door
2. PS - behind same puzzle door
Level Two - The Galvanic Butterfly Effect: 2 CC and 2 PS
1. PS - just a bit past where you first grapple with Wildvine, you'll see the can behind some crates
2. CC - behind the door that requires Cannonball to open
3. PS - in an alcove to the right of the large door leading into the first train track room (this one is easy to miss)
4. CC - after the railroad track area, you will jump up to exit. The crystal is to the right of the door. This one is very easy to miss because unless you turn, the camera angle won't show the canister at all. If you get to Wildvine's next grapple spot, you've gone too far.
Level Three - Future Mal-formed: Malware Edition: 2 CC and 2 PS
1. PS - when you unlock Gravattak, lower the bridge and walk across
2. PS - behind the puzzle door that requires Cannonball to open
3. CC - right after finding PS #2, Wildvine does a series of grapples; CC is at the top
4. CC - you will take a long elevator ride down and come to a circular area - DO NOT JUMP DOWN - the container is on a wooden platform that is very easy to blow past. Once you jump down into the circle, you can't get back up to it. This is right before the boss fight.
Level Four - Technical Difficulties: 1 CC and 1 PS
1. CC - shortly after using Cannonball to bring down a force field, you'll jump over into a little alcove that looks like a blocked-off street; the crystal is in the back. If you reach the save point, you've gone too far.
2. PS - shortly after Bloxx turns into a bridge, you'll drop down into a town-looking area; run straight ahead to get the smoothy (the canister is to the right of the Gas House). DO NOT go into the Gas House until you've collected the smoothy because the doors will close behind you, and you can't get back to it.
Level Five - Past Due: 1 CC and 1 PS
1. PS - after the first room, you'll be in a railroad tunnel; there's a ledge on the left about halfway along that's easy to run past. The PS is behind the puzzle door here.
2. CC - in the very next room, after you defeat the enemies, jump up on the left platform to find the crystal. This is another one that's not hard to miss.
Level Six - Extreme Earth Makeover: 2 CC and 1 PS
1. CC - on the stairs just before you need Wildmutt
2. CC - the first time you climb with Four Arms, climb to the left and drop down
3. PS - behind the puzzle door
Level Seven - That Was Then...: 2 CC and 2 PS
1. PS - run past first climbing area for Four Arms to area full of crates
2. CC - same place as PS #1 (you should easily see the two canisters behind the boxes)
3. CC - right after you go down in the elevator, you'll be in a train track area. You should see a green container in the upper, dead-end section. There are actually two containers here. The second is to the right of the one you can see and is hidden. On my screen, the right canister is behind the "Press Start" for Second Player. I'm quite sure this is the one I missed so many times.
4. PS - through the door that Wild Mutt is needed to open, turn left and go to the bottom of the screen; the receptacle is on the left side of the street.
At the save point, you should have ten crystals and ten smoothies.
Level Eight - ...This Is Now: 1 CC and 1 PS
1. CC - just past puzzle door
2. PS - same room where you need Wild Mutt
Level Nine - Weird Scientist: 2 CC and 2 PS
1. CC - in room where Cannonball must open door to get movable piece - it's in the closet with the movable piece
2. PS - on the right as you enter the room with the puzzle door
3. PS - in room where Wild Mutt must lead you across the electrified floor
4. CC - when you can climb with Four Arms, go all the way to the upper left and drop down
Level Ten - Plumb Crazy: 1 CC and 2 PS
1. PS - it's between the first and second sets of enemies in red boxes
2. CC - in the next set of red boxes, it's in the second box with the enemies
3. PS - you'll take an elevator up about halfway and Four Arms will then start climbing; the PS is in the next room.