That's entertainment

The obvious choice for wedding entertainment is a disco, but have you considered a band, a string quartet, an entertainer, cartoonist or magician? The entertainment is going to governed slightly by the venue size as well as their license, but having checked with your hosts, you can have whatever appeals to you.
An entertainer or magician might appeal to those who have children, to keep them occupied while the adults catch up. If this is the case you might even consider a child minder. A wedding I went to recently was slightly spoilt by the disappearance of the bride and grooms 3 year old daughter - turns out she had taken herself up to the hotel bedroom for her afternoon nap!
The string quartet is a lovely touch to compliment a venue, if you are based in a hotel maybe they could even play whilst you walk down the aisle? You can always swap to a disco later for the evening do.
If your going for a band or disco it is a good idea to get to know the front person so that they can cater to your taste in music and so they know a little on whose who in your wedding party. A few words from them to the right people will always get the party started and maybe even Grandma will be tempted to have a boogie!