With more than enough exciting new gameplay elements, Sonic Rush Adventure is considered to be the best 2.5D Sonic game.
The story takes place 2 years after the events in Sonic Rush. Sonic and Tails are caught up in a time warp during a storm, teleporting them to Blaze's dimension. They then end up stranded on Southern Island, a small tropical island inhabitated by a gang of koalas. With the help of a new unplayable but hilarious female character, Marine the Raccoon, Sonic, Blaze and Tails explore new islands, battle Captain Whisker's robotic hencemen, collect treasures and try to get Sonic and Tails back home.
Gameplay in the main stages is a lot similar to all of the previous Sonic 2D installments, but is still exciting with the Tension Feature (first introduced in Sonic Rush). Before playing on a new island, Sonic will have to plot his route on a sea chart using your stylus (similar to gameplay in The Legend of Zelda: The Phantom Hourglass) and then use one of four accessable vehicles to go there: the Wave Cyclone (a 3D waterbike racing game), the Ocean Tornado (a rail gun shooting game on a sailboat), the Aqua Blast (a 3D shoot-em-up hovercraft racing game) and the Deep Typhoon (an underwater rhythm game similar to Osu! Tatakae! Ouendan). Each ship has its unique stats and abilities (e.g. some can access certain terrains, some can go further than others). When travelling from island to island, the game automatically generates random obstacles, making the game ideal for replayability.
After clearing a stage, you will be given a specific type of "Material" depending on which stage you clear. The number of Materials you earn depends on the grade you get after clearing the stage. Materials are often required to build new ships and equipment in order to progress within the story.
The game also features new mission-based gameplay. You can unlock new missions by either talking to the koalas on Southern Island or discovering and clearing 1-act hidden islands. After you've unlocked them, you can talk to Marine and choose which mission you'd like to play. After clearing the mission, you will be given a reward (e.g. a new decoration, a new song for Sound Test, a Sol Emerald)
The boss battles now take full advantage of both screens (e.g. one part of the boss shows on top, the other shows on the bottom), but still minimal touch-screen gameplay is involved.
As usual, you need to collect all the Chaos Emeralds as well as the Sol Emeralds to unlock the final boss battle. Sonic gets to Chaos Emeralds in seven waterbike races with Johnny, a humanoid torpedo and the mid-boss of the game. He usually hides in specific locations offshore. Blaze receives her Sol Emeralds by clearing specific missions (mainly harder versions of boss battles).
Sonic Rush Adventure is the first Sonic game to have full online multiplayer gameplay, so other skilled players can race other or have Ring Battles online AND offline. In Time Attack, you can view other people's records online via Nintendo Wi-Fi Connection or even upload your own. Plus, players can do a 5-lap race on a hidden island (similar to Competition Mode in Sonic the Hedgehog 3).
The game has a total of 52 original funky music tracks composed by Teruhiko Nakagawa. Most of the music was not only inspired by funk and hip hop, but the game also includes orchestral music.
Overall, Sonic Rush Adventure is a completely original new adventure in the Sonic series, considering the new gameplay with the four vehicles along with new stages, each with their unique designs. Plus, the game takes full advantage of the Nintendo DS's capabilities making Sonic Rush Adventure one of the ultimate games for that console (despite constant button-mashing).