You could still make a cake...just because the lady makes cakes all the time doesn't mean she's automatically going to look down on yours.
On the invites, as was said before, make it clear that they don't know the sex yet. If they have a want list of stuff from Target or whatever, include the store and the info people need to acess it.
I would think that blue would be fine for a girl too...so that way they aren't limited to two colors.
The invites, you could just get some nice blank cards and fill them out that way.
Games...I can't be of much help. I haven't ever been to a shower where there weren't really stupid lame games like Guess How Big Aroung the Pregnant lady is, with string! You could just hand everyone a piece of paper with a number on it when they walk in the door, and randomly draw numbers for prizes or something. Money trees are good too, but it depends on the crowd..some people get offended if they have to get a present, and then there is a money tree too. Not that everyone HAS to put money on it.
I'd have little sandwiches, chips with dip, soda, water, maybe even pizza?, veggies with dip, cupcakes/cake. Maybe even have an actual lunch served instead, like soup and bread if it's not a huge crowd. Or a BBQ even. Not that I'm trying to make it stressful, but it'd be cool if it were more of a get together than a typical boring baby shower. If you make sandwiches, have it so people can make thier own...cuz some people are picky and hate mayonaisse/any condiments at all. People get bored fast and kinda antsy if there aren't any snacks out.
Have each mother/father there write a bit of advice, or good wishes for the baby in a journal.
For decorations, I'd just get a nice banner that says "Congradulations so and so [both the mom and dads names]" on it in any non-pastel color there is. Get some normal colored balloons. Make sure there are comfy chairs! Good Luck!