How do I word the invitation to say that there will be dinner and dancing?

Below the location line on the invitation you can say:
Reception immediately following - if it is at the same site as the ceremony,
or
Reception immediately following the ceremony
Mark's Swing Band
Glen Wood Country Club

or you can have a separate reception card like:
To help us celebrate
please join us for the reception
immediately following the ceremony
IDS Tower 50th floor
710 Marquette Avenue
Minneapllis, MN
Music by The Jay Anderson Trio

R.s.v.p. [this can be on the reception card or on a separate rsvp card]
827 Arbor Lane
Hopkins, MN 55343

There is no need to specifically indicate there will be dancing, although you could if you prefer. Most receptions have a DJ or band and dancing.