Decent shirt, tie, slacks. You should be good.
Get drunk.
Don't make jokes about how the grooms life is over now, gets old real quick. Actually, it was old before the wedding started.
Check if they have a registry and where, and get the cheapest gift. If they were dicks and registered only expensive shit, get them a card with some cash. I sure appreciated that. Give them what you can reasonably afford. Doesn't matter.
Be careful If you decide to bring a flask to a dry wedding, our venue was real fucking serious about shutting it down if they found unsanctioned booze. Keep it low key if you decide to risk it. Respect people's boring ass special day.