bus
basu
JMdict 1098390
バスbusでいこう行こうlet's go
で (by) marks the means of doing something — here, the バス (bus) as the way to travel. The verb 行くいく (go) is in the volitional form 行こういこう (let's go), which suggests or invites: “let's go”. It's plain and casual, the form you'd use with friends.
hanamaru