The all-weekend long ones we've had in Texas are typically centered around a specific event (Oktoberfest, for instance), usually on the Saturday because that's when most people can attend.
There will be a casual dinner on Friday night, for whoever is already there (we tend to get a lot of people from out of state for some reason ). Saturday during the day is the activity and depending on how many people are attending, it's done in small groups. People make their own arrangements to meet up with others for lunch. Some people wander around the event/activity together all day.
Saturday evening/night is dinner somewhere that will hold the crowd.
And depending on how late we are out on Saturday night, and when people are flying out or leaving to get home, we've been known to enjoy brunch together on Sunday morning, too.
For the much smaller not-in-Texas G2Gthat TrustedHer hosted, we went to a piano bar on Friday night, to hear TH's son play. Saturday during the day people were left to their own devices, and most did touristy things. Then Saturday night we had dinner at TH's. It was just as much fun as the big G2Gs, just much more low-key.
I think the most important element of any G2G is to give people time and a place to talk and get to know each other.