Ohanami ni ikou! お花見 に 行こう!
Pattern to remember:
place + ni + iku / ikou
Adding O or GO in front of some words makes them the honorific or polite version of that word.
More examples of how IKU/行く(go) can be used.
iku 行く = go
ikou 行こう = let's go
ikimashou 行きましょう= shall we go
ikimasenka? 行きませんか?= shall we not go?
ikanai 行かない= won't you go?
mankai 満開 = to be in full bloom