Make sure to invite plenty family and friends, and have them each come up with one or two reasons why they felt safer having this particular person fighting crime.
Have a themed party in which people dress as cops and robbers. Have games between the two with the losers have to do time in a fake jail cell. Have lots of props like a head judge and baliff and handcuffs.
Plan your theme around the Andy Griffith Show. Hang pictures of the cast, serve different kinds of pies, and keep the drinks in a cell you lock yourself in.
There are a lot of really unique things you can do. As for decorations you should get pictures of the officer from when he first entered the force and have pictures of him up that will show his career progress.
When I retired, the other "first" policewomen threw an unofficial party, overlapping two shifts, and we drank a lot. I was given a gold medallion and thrown in the pool.
