Activate Timstream Navigator by paying {2}{U}{U}, tapping it and putting it on the bottom of your library, causing you to take an extra turn after this one.
Activate Riptide Shapeshifter by paying {2}{U}{U} and sacrificing it, naming any of Timestream Navigator's types, and then revealing Timestream Navigator from your library and putting it onto the battlefield.
Move to your next turn.
Cast Riptide Shapeshifter from your graveyard by paying {3}{U}{U}.
Repeat.

Combos with "Riptide Shapeshifter"