Three things impose faster battery drain:
1) Network activity, especially over a cellular data network
2) GPU activity, typically by 3D games
3) GPS activity, typically by maps or other location-aware apps.
You say it drains even on airplane mode so it's probably not #1, but that won't affect #2 and #3.
A well-designed app will not use battery-intensive resources when it isn't in the foreground, but not all apps are well-designed. Make sure you *really* quit (not just background) any apps or services on your phone which might be using the GPU or GPS when you don't need them.