Perpectively-speaking, it's feels like we can see the left side, front side, and right side all at once.
A potential solution is to think of each platform as its own three-dimensional object, facing at all sorts of different angles.  But never should we see two adjacent sides of the same object (because when in nature can you see both the front and back side of something?)

Probably you're puzzled because you're thinking of the platforms as perfects parallelepipeds, but that's not what they're meant to be in the game - here's a possible interpretation seen from above:

I tried hard to find a way to add highlights, to make the platforms prettier and, at the same time, not to lose on the color side. I even tried weird things like this:



but, eventually, I settled with the simple highlighting solution indicated in a previous post (actual screenshot):



While at it, I retouched again a couple of clouds slightly.

What do you say? Time to move on?

Update: I made a few minor changes to the clouds (accentuated a few curls) and implemented the code - actual screenshot:



(The jerks are due to the time that passes between the instructions poke53272,gp : poke53265,91, where the first changes the video memory location and the second the video mode. I've already managed to hide them by making them occur during the horizontal blanking, but the solution isn't 100% stable, so there's still work to do. EDIT: fixed, now.)

I've also been thinking about highlights...

The first solution that came to mind was to turn the dents into bumps and thus use the 3 background colors for both them and the highlights; yet, this idea didn't convince me at all.

The second solution was to just turn the dents by 180° so that they'd show the bright side, which could be painted in the highlight colors: however, a quick mockup (which I trashed, sorry) didn't manage to convince me.

The third solution came from an old idea that actually got implemented in the first stages of development:



It surely makes things more interesting, but for some reason the slanted top side doesn't really seem suitable to me (and it also costs the loss of gray as shadow color). What do you say?

EDIT: alternative (and better) highlights:



#1 requires only 1 character, the others need 3 characters and more processing time (but those are hardly problems). I can't really decide, probably the best - as it often happens - is in the middle

Also, I've been considering (not for the first time) to remove the outlines, but they are too important both for the cartoonish feel and the clear separation of the platforms.
A final thing to note is that I have now 13 characters free (courtesy of the bitmap top): maybe they could be used to improve the platforms (f.ex., they could be used to add dents also to the slanted sides)...

EDIT 2:

Just tried a different approach:



The problem is that I can't figure out how to enrich the platforms with the 4 colors used for highlights and outlines (white, the two lighter grays and black)

EDIT 3:

Some more ideas:



Only the first looks acceptable to me, but I don't know if I'll go for it as the highlights make the platforms seem too smooth (and there's also the loss of gray for dents that bothers me). Ah, please ignore the bottom-right one: it isn't tecnically possible (that's what happens when one doesn't sleep enough)

Why the hell are you doing this in BASIC?  

Long story made (really) short...
The more time passes by, the more I wish I had a simple machine to play around with, hitting directly its hardware. The C64 is a machine I love and, unfortunately, know too little about, so, at some point, I decided to have some fun fiddling with it. I had even a clear idea in mind: I wanted coloured HIRES sprites. I knew BASIC would be enough for that, so I chose it to get immediately started. I ended up doing much more: I created a custom framework that allows pushing BASIC to wild limits (thanks also to the BOSS-BASIC compiler) and that provides a large and comfortable library of functions of all kinds. Then, during one of those nights of brainstorming (which actually started with the focus on that other game I told you about privately a while ago), I had the crazy idea the game is based on and so I started working on it knowing it would be feasible. Eventually, the fun became a (pleasant) challenge: to create a colorful, 100% HIRES game using just BASIC.

The new sky looks nice (maybe a bit too straight),

Indeed, I should break those horizontal lines more. Here's an attempt (which also tries to re-arrange the clouds in a more effective way):



(I just noticed one thing that bothers me: the similarity of the left "nose" of the clouds. I guess I'll change that of the clouds closer to the sun).

but I agree with ptoing, the sky is a bit too busy and is drawing attention away from the playing area. The highlights he added actually help prevent that by making the platforms pop out more, drawing your eye down again. (And of course his sky is less busy, partly because of the muted clouds [though they don't look very nice] and partly because of the lack of sun.)

Yes, the highlights would help in that sense, but they come at a cost, and the result is that neither the highlights nor the platform look perfectly nice.
But, no worries: as said above, the outlines attract a lot the attention, not to mention the fact that the objects (which the player has to catch) move, whereas the rest is static. (An additional word for the sun: it doesn't contrast highly with the sky and, as it can be seen by trying the playable preview, it isn't much of a problem - or a problem at all ).