At first glance I thought 'a nice trick' meant a trick that works really well because it catches someone properly by surprise and gives them a nasty scare / burn / sweet / bruise / insect / death. I'm not a fan of practical jokes, and haven't attempted one since I almost blinded my cousin John with the 'look down the hose and see what's blocking the water' trick in 1974, so I was pleased when I noticed nice was in italics so the idea is to make someone smile. But that still means you have to make them vulnerable in order for the nice bit to work. I'm inexperienced at this, and worried I might misjudge it:
Me [in hired police uniform, knocking at door of acquaintance's house]: Are you the wife of David Thomas Saunders?
Mrs Saunders [suddenly pale and shaky]: Yes
Me: Did he leave home in a red Vauxhall Cavalier this morning?
Mrs Saunders [holds the wall, legs crumble]: Oh my God
Me [producing daffodils from behind back]: Well he's got you these!'
I'm going to see if I can come up with a trick, or if anyone suggests one – otherwise I may have to interpret it as 'Give a stranger a nice surprise'.