Bonus stages - Successfully complete the indicated task in Planet Superstars mode to unlock the corresponding stage:

Alex Kidd: Successfully complete "Get A High Score!" in the "Space Harrier" stage.
Golden Axe: Successfully complete "Speed Rings" in the "Sonic The Hedgehog" stage.
NiGHTS: Successfully complete "Monkey Ball Keep Up" in the "Super Monkey Ball" stage.
Outrun: Successfully complete "Hit Justice Shots" in the "Virtua Squad" stage.
Puyo Pop Fever: Successfully complete "Doubles Match" in the "Super Monkey Ball" stage.
Space Harrier: Successfully complete Tournament Singles in the "Space Channel 5" or "Virtua Squad" stage.
Virtua Squad: Successfully complete "Hit The Big Zombies!" in the "Curien Mansion" stage.

Bonus courts - Successfully complete the indicated task in Planet Superstars mode to unlock the corresponding court:

Golden Axe: Successfully complete all Super Monkey Ball missions.
NIGHTS: Successfully complete all Jet Set Radio missions.
Outrun: Successfully complete Exhibition Doubles in the "Outrun" stage.
Puyo Pop: Unknown.
Sonic The Hedgehog: Successfully complete Exhibition Singles in the "Sonic the Hedgehog" stage.

Bonus characters - Successfully complete the indicated task in Planet Superstars mode to unlock the corresponding character:

Alex Kidd: Successfully complete Tournament Singles in the "Alex Kidd" stage.
Amy Rose: Successfully complete Mission 2 in the "Sonic The Hedgehog" stage.
Gilius Thunderhead: Successfully complete Mission 1 in the "Golden Axe" stage.
Gum: Successfully complete Mission 12 in the "Jet Set Radio" stage.
MeeMee: Successfully complete Mission 8 in the "Super Monkey Ball" stage.
Pudding: Successfully complete Mission 1 in the "Space Channel 5" stage.
Reala: Successfully complete Mission 2 in the "NiGHTS" stage.
Shadow the Hedgehog: Successfully complete Mission 14 in the "Sonic The Hedgehog" stage.

Bonus ball games - Successfully complete the indicated task in Planet Superstars mode to unlock the corresponding ball game:

Puyo Pop Fever: Successfully complete "Get A High Score!" in the "Puyo Puyo Fever" stage.
Space Harrier: Successfully complete "Get A High Score!" in the "Space Harrier" stage.
Virtua Squad: Successfully complete "Get A High Score!" in the "Virtua Squad" stage.

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Play Ball (10 points): Win your first match.
Starting Grid (10 points): Win with all 8 starting characters.
Beat them All (30 points): Beat all characters in Singles matches.
Original Teams (30 points): Beat all the classic Doubles pairs.
Cameo Appearance (20 points): Hit a Sega court cameo.
To The Max (20 points): Hit 3 Max serves in a row.
Seeing Stars (20 points): Enter Superstar mode 10 times in a match.
Sega Rally (20 points): Hit a 40 shot rally.
Top Gun (20 points): Hit 100 aces.
After Burner (20 points): Hit a serve over 100mph / 160kph.
OutRun (20 points): Run a total of 10 kilometers / 6 miles.
Step into Sega World (10 points): Complete your first Challenge (Rank A).
Classic Complete (20 points): Complete all Challenges in one Sega game (Rank A).
Challenges 101 (30 points): Complete all Challenges in all Sega games (Rank A).
To be this good takes AGES (60 points): Complete all Challenges in all Sega games (Rank AAA).
Full Roster (30 points): Unlock all Secret Characters.
Play the Field (30 points): Unlock all Secret Courts.
Score Attacker (30 points): Unlock all Ball Games.
Magical Sound Shower (30 points): Unlock all Secret Music Tracks.
Climb the Ladder (20 points): Win the final stage in Tournament mode.
Secret Stage (30 points): Beat the hidden stage in Tournament mode.
Mouse Mania! (20 points): Rescue 500 ChuChus over time.
Zombie Hunter! (20 points): Hit an 8 zombie combo.
Master Tagger! (20 points): Clear a Tag without missing a shot.
Puyo Puyo! (20 points): Clear a 20 Puyo combo.
Super Sonic (20 points): Collect 7 Chaos Emeralds in one game.
Dragon Slayer! (20 points): Destroy the Final Squilla in under 20 seconds.
Pot Black! (20 points): Clear 4 Trick Shots in one game.
Supercop! (20 points): Destroy Kong without being hit.
Party Time (30 points): Play each type of Ball Game Multiplayer.
Surf the Net (10 points): Win a Match on Xbox Live.
Top of the Tree (20 points): Win a Tournament on Xbox Live.
Ball Buddies (30 points): Win 5 Xbox Live Doubles matches.
Top Ranker (20 points): Win 10 Xbox Live Ranked matches.
Let's Get Friendly (20 points): Win 10 Xbox Live Player matches.
Space Channel 5 (10 points): Watch a match on TV.
Swinging Report Show (30 points): Become a TV Superstar.

Additionally, there are eight secret achievements:

Thunderhead (20 points): Unlock Gilius. His brother was killed by Death Adder.
Rose (20 points): Unlock Amy Rose. There's no hiding from her, so pink and upbeat. Such a tough cookie and so sweet.
Nightmare (20 points): Unlock Reala. Only in your wildest dreams.
Sonic Rival! (20 points): Unlock Shadow. I am the world's Ultimate Life Form.
Stuck To Your Blades (20 points): Unlock Gum, An original member of the GGs.
Piece of Cake (20 points): Unlock Pudding. Get your coverage from Channel 42.
Jan-Ken-Pon! (20 points): Unlock Alex Kidd. For seven years he lived on Mount Eternal studying the ancient art of Shellcore.
Eei-Eei-Poo! (20 points): Unlock MeeMee. A stylish monkey who is adored by all other monkeys.