Orb in the Good Future
This is one of the glowing orbs from the Good Future.
Good Future Preview
This is the Hidden Palace Zone Good Future, it is far more finished in released versions than in the screenshot.
Solitary Green Orb
A green orb high up on a cliff.
Emerald Shard recipe
To access Hidden Palace Zone, you need to build a portal out of polished granite, and then ignite it with the Emerald Shard.
Badniks
Not all of them are from Sonic 2's Hidden Palace Zone. Some from other classic Sonic games are there for variety's sake.
Orb and water
A glowing orb next to the water at the bottom of the cave.
Blue Orbs, Platform and Water
You can see the blue glowing orbs, the palace floor, and the water at the bottom of the cave.
Platform
The palace platform, visible higher up away from the water.