The only thing that your guests will remember is whether they were welcomed, fed and watered, and if they were comfortable. Everything else is largely irrelevant. With that in mind -
Make sure you have plenty of canapes and that these are ready to go as soon as everyone has their first drink. There needs to be enough for a good few passes round the room, bearing in mind that some people will hoover up 5-6 in one go.
Make sure that there are plenty of bar staff - nothing worse than long queues. Plus lots of non-alcoholic drinks on offer and something suitable for kids if you are having them there - e.g. ribena, fruit shoots etc. Make sure the bar takes cards and contactless payments - if it is cash only then make sure people know about this well in advance of attending!!
There needs to be plenty of seating available if you aren't formally seating people for the dinner. Make sure there is somewhere to stash coats and bags and that there's enough room for buggies if guests have young kids with them.
As PP Have said, intervals for food should not be more than 3 hours - 4 maximum. Drink makes people hungry!
Music needs to cover off something for everyone - again as PP have said, DJs who'll intersperse current music with some cheesy hits will get a good mix of people up dancing. I've been to loads of weddings and the only one where nobody danced is the one where the DJ refused to play anything other than really niche hip-hop - and not the family friendly radio edits either.
Make sure you take the time to go round each table or group and have a chat with everyone. People will remember the fact that they were made to feel welcome. In years to come nobody will remember the colour of the barn, or the details of your dress, or whether you had table favours or not. But they will look back and recall if you were welcoming, and whether they had a good time or not (99% of which is dependent on being fed, watered and comfortable!).
Personally I wouldn't bother with extras. But if you want to do something I'd go for a photo booth as they are usually popular and you can get some nice memories of bits of your wedding that you weren't part of.