This past April was our 2nd cruise. Both our trips had been 5 night cruises. We returned to our room one day to find envelopes with the invitation and a nice Carnival tack pin with the ship name on it. The invitation was to the Past Guest Party.
As previously noted, it was a party for about 1 - 1 1/2 hours where the staff welcomed you back, served snacks and drinks (and you can get whatever you would like at the bar - enjoyed a delicious Strawberry Daquiri from the bar). They did do a slide show and the kids were invited and attended as well.
Enjoy your planning. Glad to hear you have booked again.