Just want to ditto what has already been said, and I'll give you an idea of how our UU church runs, although they can all be different.
We have a kinda official service at our uu church. I say kinda because it's not at all a 'serious ceremony' or anything, but we do have a ritualish feel to it. First we light the chalice and say the thingy about being a welcoming congregation, then we have a reading (usually an inspirational poem, paragraph from a book, spoken song, etc). We have a greeting to those sitting around us, where we all say howdy for a minute or so (usually the pastor has to corral everyone back to their seats LOL). We have Candles of Joy and Concerns, where people can, if they want, come up and light a candle and share (if they want) a joy or concern with the congregation. There is always one last candle lit for joys and concerns too personal to share. Then we 'sing out the children' as they head down to RE (Religious Education) class (more on that in a sec). We usually follow this with a time for meditation, sometimes with music, sometimes with a reading, and sometimes just quiet introspection. We have the offering, and announcements are read regarding upcoming events. The pastor gives a sermon on whatever the topic is, and afterwards he passes around a basket asking for questions to answer. Some are serious and deep theological questions, some are benign
(what exactly do you want us to call you? Pastor? Reverend? Smitty? He laughed and said Your Holiness would suffice. Everyone else then laughed and promised a tar and feather party after social hour lol) But regardless it's a neat time for us to interact during the service. After the service we have about 15-20 minutes of 'social hour' before the kids are done, so the adults can mingle without interruption. Then the kids come up (there's snacks upstairs with us...always a motivator LOL) and we head home.
Religious Education for the kids is actually nothing of the kind. Last year they explored a different ancient religion each month (Egyptian, Greek, Roman etc) and this year they are exploring "What Can I do for UU", which is obviously a play on words while emphasizing social justice

There are also lots of other social things going on during the week and each month. There's a social justice committee, a GLBT action committee, an environmental committee, and more.
If you've found a UU church w/300 members, that's bigger than ours so is likely to have more to do. We love our UU church because UU's believe in "Deeds Not Creeds" and I never have to worry about my kids coming home with wacky ideas like the ones they got over at Gmas church

Hope that helps!
Bellevuemama