The icy future is significantly shorter and easier than Crystal Path, The Shadowlands (which is required to complete before even travelling to the future), and even The Lunar Cheese Extraction Facility. The main reason for this is because it is a remixed version of Pinewood Heights, the game's first major area, and therefore the obstacles are not a major threat other than the increased number of spikes. It also helps that the only ball you can use there is the ice ball, which has pretty good handling.
Dr Cliché's Underwater Lab at the end of the game is challenging, but nowhere near the difficulty level of some of the other levels and is fairly short once you know what to do. There's not even that much to it once you've unlocked the combination lock. You just enter the next room, avoid a few electrified dots to press the switch to the bomb room, then go into it and automatically finish the game.
Nightmare Fuel: The future. Normally you don't notice just how much animation is there and how lively the game world is. When everything is frozen and stopped the effect is pretty damn frightening. The music and the animals frozen on the spot are especially unnerving.
The Crystal Path. Features some tricky puzzles, and the glass ball is required for most of it. The crowner is probably the switch that catapults your fragile form across the room when you touch it.
The Shadowlands. It's a major trial-and-error maze, and the platforms often fake you out.
Markstone Ridgemine. Every time you mess up, you have to start again from the beginning. That music gets old fast.
The Lunar Cheese Extraction Facility. It's not as tough as Crystal Path or The Shadowlands, but the big, long jumps with fans pushing you back are difficult, while the underground section has very narrow corridors with pits everywhere.