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It's time to talk about Sonic CD stages! (some of this courtesy the GHZ and SonicRetro, because they're insanely useful for this stuff)
Most sonic games have it: the requisite first stage full of plants and greenery we know and hopefully love, complete with cheerful music. Palmtree Panic is the one here, with triangular ground patterns and grassy floor reminiscent of just about every single Genesis/Mega Drive sonic game. It's also a great stage to introduce the concepts of Past and Future and what they mean for the entire game.
- Past: The world takes on a washed-out hue, giving a bit of a prehistoric look.
- Bad Future: Grey, purple and orange landscapes make the stage look polluted. Tubes and metallic plants have taken over the environment.
- Good Future: Even brighter than the present, the good future shows metallic plants, tubes and machinery in harmony with the environment and animals.
The stage features a gigantic loop at the beginning of the first stage (showing off background flipping as Sonic keeps going up) and small rocky platforms which look like Donut Platforms from mario. These are stationary in the Past, stationary but invisible until you step on them in the Present and will fall out from under you in the future.
Finally, Amy waits at the end of Present Zone 1 and will cling to you if you slow down enough. Aww, how cute. 
That's about all she does, though, which makes her infinitely less annoying than any of her other appearances.
BADNIKS
Mosqui (mosquito)
Flies around in a straight line until Sonic walks underneath it, where it will attack by diving down and jamming its nose into the ground. The damaged version has a crooked nose and flies slower.
Pata-bata (butterfly)
Flies around in a zig-zag motion. The damaged version has a hole in the wings and flies slower.
Anton (ant bot)
Putts back and forth along the ground. The damaged version looks depressed 
and moves slower.
Kamemusi (beetle bot/stink bug?)
This beetle bot rolls back and forth along the ground and sometimes shoots projectiles from its rear. It looks the same in its damaged form, but rolls slower and cannot fire. This is also one of the many badniks which has no name in the manual; its name is apparently found in the source code. Consistency!
Taga-taga (crab? lobster? water bug?)
Hops in and out of waterfalls, like piranha from other Sonic games. Found only in an intact form.
BOSS: EGG-HVC-D01
The EGG-HVC-D01 is a mech walker with spikes on its feet and two bumpers on its arms, repelling almost all attacks while they're extended. The boss will step back and retract its bumpers when getting ready to charge forward, leaving itself open to attacks. The first two hits to the cockpit will each knock off one of the bumpers, and the third hit will finish the boss.
Or you could get hit and run behind it. That works too.
The Bad Future version is grey and red, while the Good Future version is... bluish and pink 
. I don't even know.