There are numerous pubs in the Grand Canyon and they are all very busy but the main early evening entertainment is watching the sunset. On the North Rim, the main bar is actually right on the rim so you can actually do both at once but getting a seat is a go early affair. South Rim pubs are more in the hotels, which are set a bit back so you'll have to hit one of them after or before the sunset. This is one of those places where the low light of early evening makes a big difference on what you experience visually. There are many lookouts from which to do this as well as numerous little nooks and crannies along the rim walk. Due to the popularity of this activity, it's nearly impossible to find a place all to yourself. Obviously, some of the best viewpoints are incredibly crowded. While it's nice to take one of those in at least once, if you have more time, try to get to a more secluded spot once too. You'll have to drive a bit away from the main area but it's well worth it. It's a good idea to scope some out during the day to find potential ones with a good view of the canyon. Then you can drive out late afternoon and stake out your territory, waiting for the glow to begin.
Dress Code: Remember that you are at 8000 feet and once the sun goes down, it gets chilly. Wear warm clothes and even a hat if you want to be comfortable. Bring some snacks and a couple beers to make for a perfect low cost evening.