There are four places in the Keokuk- Hamilton area to collect fine quality geodes. Shefflers, St. Francisville, Jacobs and the Fox river. Probably all in a 20-30 mile radius. I've gone to the geode festival (late September) in Hamilton Ill. across the Mississippi river from Keokuk Ia. the last three years now. Jacobs has many small golfball size geodes there. Small but very good quality and easy to get.
St Francisville has the larger ones. Not easy but many are over a hundred pounds. I personally prefer the Fox river. The lower the river level the better. Just walk the river bank. They are different sizes all over the place. Some you can pick up, some partially exposed, you may need to dig. Summer heat can be brutal and the river is refreshing to wade in, but watch for snakes and poison ivy. Didn't do well at Sheffler twice. Except for St. Francisville, landowners charge $20 per 5 gallon bucket. St. Francisville gets an extra .55 cents a pound. That can be expensive! None allow on sight cracking.
Google "first cracked geodes". They are the club that host the festival. Website could be better.
Unless its a family thing, I wouldn't consider the diamond mine. Perhaps fun for the novice, but not fruitful. Go an hour or so north to the Hot Springs area for some clear quartz crystals. Most mines are $20 a day for all you can take. I'll be there next month.
Enjoy, Ray