Retreat to Coralhelm + Dimir Aqueduct + Llanowar Scout

Steps

1. Activate Llanowar Scout by tapping it, putting Dimir Aqueduct from your hand onto the battlefield.
2. When Dimir Aqueduct enters the battlefield, it and Retreat to Coralhelm trigger.
3. Resolve the Dimir Aqueduct trigger, returning itself from the battlefield to your hand.
4. Resolve the Retreat to Coralhelm trigger, untapping Llanowar Scout.
5. Repeat.