I tried myself to see if there was anything I could practice on before I actually got my login details, but there wasn't. So my first 2-3 weeks I wasn't really doing much work! Although my manager is actually away at SAP HQ this week on a training course, so you might be able to do the same thing? I don't think there is any way to demo the software yourself because it would rely on quite a lot of variables to actually do anything meaningful. That said, if you manage to get a job at a company who uses SAP, you can practise in a training environment so that you don't screw anything up!
I got my job by pure luck. I had interviews for loads of big companies and didn't make it through any of them. When I started giving up, my university were applying to places without even telling me. My current manager liked my CV because he graduated on the same course and university as me 15 years prior, and I had some decent coding background. His workload was piling up and he thought it would be a good chance to introduce a student to the workplace, as well as (eventually) helping to lessen his workload. My pay packet is pretty nice too - more than your average graduate salary! And if I decide to continue down the SAP path when I graduate (I could go down the accounting route), my manager has sweetned up the idea by boasting that in his peak he was on £1,000 a day (as a contractor)! Very tempting and rewarding if you can be bothered to learn SAP properly..