I've attended 26 ukulele festivals/events/camps as a participant in the past several years. Some have been outstanding, some merely fun. I've never attended a bad ukulele festival.
What I've discovered, though, is that what I will especially enjoy, others may not find so great (e.g. I don't really like open mic--either to watch or participate in--but for some people, that really *makes* a festival super). The converse holds true, of course.
What you can typically count on when attending an ukulele festival are playing workshops (except there are *none* at Hayward's Northern California Ukulele Festival), the chance to kanikapila/jam with others, performances, vendor booths and the chance to socialize. Some also have cultural or craft workshops (great for keeping accompanying spouses occupied, or not), open mic opportunities and food vendors (ahhh…the malasadas at Hayward…).
My favorites for really "learning" are the camp-type experiences where you're with the same instructor for three or four days in a row (the Portland Ukulele Fest was one of those). It's really concentrated growth.
My favorites for having fun, schmoozing and catching up with ukulele friends online and otherwise are the shorter festivals--and I take care not to book up my full day with workshops so I have time to do that socializing.
If it's live performances you want, follow the ukulele players you like and see which they'll be attending (they'll typically teach workshops there, too--but I guarantee you from experience, just because they perform well doesn't mean their stellar instructors, although they may be!).
If you're really looking to sample lots of ukulele from different luthiers and makers, set your sights on a festival which will have a high attendance; it's expensive for the vendors to travel with all those instruments and they're more likely to attend festivals where there will be lots of folks with credit cards to purchase their goodies. You'll usually get the "local" luthiers to show up at the smaller, regional festivals as well so if there's a luthier you like, find out where he/she will be exhibiting and go, even if it isn't one of the "big" uke events.
I've been writing up accounts of many of those ukulele festivals over the years;
go here to read about them and see some photos. My goal is to share with folks the great joy, fun and learning available at ukulele festivals. Maybe it'll convince you to attend your first one!