Engagement parties are typically causal affairs, and even couples planning formal nuptials usually prefer laid-back get-togethers leading up to the big event. Themes are optional, and they're often related to the proposal. If he popped the question on the seashore, beach décor and seafood hors d' oeuvres make the perfect party accent.
Be sure to supply your hostess with a guest list. Anyone you include in your engagement celebration will also expect an invite to the wedding. Your great aunt who lives out of state probably won't feel snubbed if she's excluded from your engagement party, but things could get nasty if you invite her to the opening celebration and not the actual wedding.
Some guests may want to bring gifts. You don't have to set up your bridal registry in advance of your engagement party, but you can at least pick out your china pattern, silver and glassware ahead of time. Let the hostess know, and she'll get the word out. If you're hosting your own celebration, don't advertise your registry unless someone asks.