Sonic Mania

Green Hill Act 2. Grab the Fire Shield (hidden behind a waterfall), then walk onto the spinning wooden log bridge while wearing it - the flames set the bridge alight.

Hit a Star Post at full running speed so it keeps spinning on its own - it needs ten rotations from the one pass. Build up speed first (a downhill stretch or Speed Shoes helps) and blast straight through; don't jump back into it, the single fast hit does it.

Studiopolis Zone. Find a Shutterbug (the flying camera badnik), stand in a safe spot inside its frame and let it photograph you ten times - each flash counts. Just don't get hit while you wait.

Lava Reef Act 1. Climb into one of the Walker mechs - the striding leg-platforms that carry you across the lava - and use it to crush a Rexon, the lava-serpent badnik that rears its head up. Kill one while riding the Walker and it pops.

Easiest in Green Hill Act 1. When the act ends, the Goal Plate drops from the sky - jump into it and knock it around in the air before it lands, and it spawns a hidden item monitor at a fixed point. GHZ Act 1 has two such points (between the purple flowers on the left, and under the yellow sunflower on the right), so bat the plate about and a single ending can uncover both boxes.

Hydrocity Act 1. Find and ride every little boat dotted through the level in one run - hop onto each one you pass. Miss any and you'll want to replay the act (Stage Select / Time Attack) for a clean sweep.

Beat the game with any character, any ending - it pops on the credits. You'll get this naturally on your first full run.

Two ways in: at the Chemical Plant Act 2 boss (a forced game of Dr. Robotnik's Mean Bean Machine), or in the standalone Mean Bean mode from the main menu. Line up beans so one drop clears three groups in a row for the 3-chain combo. Mean Bean mode is the calmer place to set it up.

Flying Battery Act 2. Take the upper route toward the end of the act to reach the teleporter windows on the airship's hull, and jump through one - you fly clean through the glass.

Collect all seven Chaos Emeralds from the UFO-chase Special Stages. You reach a Special Stage through a Giant Ring hidden in the levels - grab the emerald in each. Seven in total unlocks this and the true ending. Stage Select's "?" panels let you retry a Special Stage if you fall short.

Mirage Saloon Act 2. Find the wooden barrel and push it off its ledge - riding it down drops you to a hidden Giant Ring (the Special Ring) tucked off the main path, which warps you to a Special Stage. Mind the barrel's momentum so you land on it.

Press Garden Act 1, playing as Sonic. Grab the Fire Shield, then use its dash to blast into the hidden room and smash every breakable crate inside - the Fire dash is what clears them, so Sonic is required (Tails and Knuckles can't do it).

Metallic Madness Act 2 boss. The boss dispenses little Eggman robots (Eggmins) from a gacha-style dial. Let it churn out until eight are on screen at once, then wipe them all out together rather than picking them off as they appear.

Oil Ocean Act 2. Take the lowest route and dive down into the oil between the swinging pendulum platforms near the end of the act - a hidden submarine sits at the bottom.

The Blue Sphere bonus stages (entered through the Star Post rings). Clear all 32 to at least silver - silver just means completing the stage, i.e. turning every blue sphere. Grind them from the Star Posts, or via Stage Select's bonus panel once unlocked. Push for gold (No Way? No Way!) and you bank silver on the way.

Titanic Monarch Act 1 with no damage - reach the end without taking a single hit. Going in as Super Sonic trivialises it (contact drains rings rather than ending your run), and hits taken by a co-op Tails don't count against you. Otherwise it's a learn-the-act patience run; use Stage Select to attempt it clean.

Stardust Speedway Act 2. Reach the tower at the end of the act (where the Metal Sonic race begins) in under a minute - a route-and-momentum run. Keep your speed up, take the fast upper lines and never stop; a replay or two to learn the layout makes sub-60 comfortable.

The gold-medallion tier of the Blue Sphere stages: clear all 32, and in each also collect every ring (turn all blue spheres AND grab all rings for a "perfect"). Gold is the stricter standard, so it supersedes Full Medal Jacket - go for gold and silver comes free. This is the game's biggest grind.