The first thought that occurs to me is that you are like two objects attached via an inclined plane wrapped helically around a central axis.
Ok. With a crowd this size you cannot depend on the value of the card being important at all. No one will be able to see it it clearly, and you'll lose the impact of the reveal because people will have to wait for it to be confirmed. Unless you're really good at getting the volunteer on stage to convey the emotion to the audience.
Mac King does a cards across that's absolutely hilarious, but if you copy him you'll clearly be copying him. You can take inspiration from that, though.
Book tests are awesome, when done right. They're downright boring when done by most people, though. Mentalism in general is much more difficult than most people think it is, which is why most of it out there is horrific and dull. Sheer Luck is one of those that can be done pretty well without having to hone it for too long.
Predictions have to be delivered well, and again, most people don't deliver it well. Watch Derren Brown's reveal in ... Evening of Wonder I think? The long scroll at the end of the show. Excellent Reveal. Max Maven has done this great reveal of a prediction where he basically says, "This big envelope has been in view the whole time. I told you it's a prediction. If my prediction is correct, then I shouldn't even need to touch it to show you, should I? In fact ... I don't even need to be here." And then he walks off stage and let's the audience decide what to do. That is not an easy thing to pull off, though.
PK Touches is a great routine. I've done a version for years. It only took me about 6 years to really fine tune it to be the showpiece it is now. You can learn the basic version in a few minutes and probably have that rehearsed to proficient execution in a day. However, that will fall flat. You need to learn how to deliver a script that connects to an audience before that routine will really hit hard.
A little less than 3 weeks is a very short time to try to learn anything new. My biggest piece of advice is look at what you already know and do very well, and see if there's something you can take and have it conveyed to someone who cannot see what you're holding.