There are games you can play (there is a thread on that around here somewhere). I've been to some where they have heaps of games (but they seem a bit full on) and others where everyone just sits around, opening pressies and having a coffee and cake. Would your SIL be comfortable with loads of games, or a more informal approach? A lunch sounds like a great idea, as is the cake.

Some of the games can be;
the fastest person the put a nappy on the baby (doll)
Guessing the sex and weight of the baby (a bit anti-climatic because you don't know who won until after the event)
Put a heap of pegs ona bit of rope and the person who can take off the most pegs and hold them in one hand wins.

There are heaps of others too. try googling it.