I have a small n-scale switching puzzle I take to our monthly club meets. It's is completely battery powered using nothing more than a 9V battery. I am using a DPDT switch to reverse directions and a simple potentiometer for speed. That's it. I can only run 1 engine at a time but for a puzzle that's fine. That may also be fine for a coffee table display layout that either operates continuously or even for one person to switch with.
The 9V battery isn't much juice. Depending on what engine I use, it may be adequate or it may be a crawl. Again not much of an issue for a small puzzle. However this doesn't mean that I couldn't get more speed with another type of battery. Voltage will determine speed. Amperage will determine how long that battery lasts. A higher voltage battery may work well. There is a wide variety available in power tool batteries. You can get 12V, 14.4V, 18V, etc. Obviously there is a limit that you don't want to go over but the point is that batteries are easy to use and quite plentiful. Power tool batteries are also rechargable and you could have 2 with one charged at all times. The small draw in n-scale will be minimal and a charge should last a very long time. If you want to run multiple engines in a consist, you can with these higher voltages (higher than my 9V).
If you want to be able to run more than one train at a time on 2 separate loops, that should be fine too assuming you want to keep them doing the same job. However if you want to be able to control 2 trains independently you'll need to go back to block wiring with toggle switches and have 2 batteries. Just something to think about. For a coffee table I'd consider it as you won't have to worry about a power cord and odds are that the layout in this small of a space won't neccessarily be intensive enough to justify DCC. It's definitely a cheap and easy option.
Multiple batteries would be an option and rechargable tool batteries would be great. For your little layout remember that batteries in series will raise the voltage, but batteries in parallel will raise the amperage. You could run 4 9vdc batteries in a series parallel circuit and have the same power as if you were plugged in.