I've gone the last two years, mainly because I really like Bad Durkheim and it's only 20 minutes from where I live. I usually go early and have fest food as lunch, but it really gets crowded after4pm, especially Fridays and Saturdays. Sundays it's more like the place to hang out for families in the afternoon. Most of the booths won't open before 1100, even on the weekend. It is a big fest, not Munich Oktoberfest huge, but big. They fill the same space at Christmas with the market (and the side streets) but very few tourists think about the town as a holiday destination in December.
Can you spend a day there? It depends. To me the rides are not a big deal, and I usually spend just as much time walking around the town as I do at the fest. It's better when there's a group, and you can get a table and relax. Do get out and look around the town. They have a huge "saline" which is an ancient, and effective, way to control temperature and humidity in the vineyards. There's a nice old town just up from the fairgrounds, and several truly excellent places to eat. There's a casino, a couple nice gardens, and just about every town up and down the line is very much in the wine business. It's a lot like Napa (where I also have lived).
There's beer, there's always beer in Germany. And food will range from grilled salmon, to wurst (many kinds), candies, nuts, gingerbread, ice cream, potatoes multiple ways, and a lot of it will be stuff you never see in the States. The wines are local, this is one of the biggest wine regions in Europe, and you can get it by the glass or pitcher/bottle. They also drink the schorle, which is a local thing, a half liter of wine mixed with sparkling water (think wine cooler from the 80's). It's very nice on a hot day, but can sneak up on you, which is why I never stay past 8pm. Drinking starts around 11am.
If you stay in Mannheim it's a bit tricky to get there by train, but you need to change in Freinsheim, which is another, even smaller, town worth getting off and looking at. It's still got the entire wall from back around 1200, some of which is still lived in, and it's got some good places to eat also. You also change in Frankenthal, which as a good castle if you want to climb up there. (You'll also pass Castle Frankenstein on the train).
But I recommend you not stay in Mannheim, which is an industrial type city, and can have it's problems. Your son may be better served if you stay in Heidelberg, just a few minutes direct ride on the train, as that city has a very active university night life, and tourists. Or, if you want more of a quiet place, consider Neustadt (on the wineroad), which is a direct line to and from both Mannheim and Bad Durkheim.
This is an aerial of most of the fest:
[img]https://i.imgur.com/uTR9WS3.jpg[/img]
The wine garden:
[img]https://i.imgur.com/gRUKxLa.jpg[/img]
Prices from 2023. i didn't notice much difference last year:
[img]https://i.imgur.com/SPnq3y6.jpg[/img]