@OP: Unless you are planning on using a lot of buses, I think you might be best served by buying individual tickets for each subway ride. If you have coins or clean bills (5s, 10s, 20s) on hand, you can use the vending machines. Otherwise, you can buy tickets at the windows. As crowded as Beijing's subways are, the process is fast/easy. I'm hard pressed to think of any compelling reasons for people visiting for just a few days to bother with yitongka. In fact, most people I know who live in Beijing, but don't rely on public transportation get to/from work, don't bother with them at all.