One of my favourites is to have departure weather not suitable for return, then have the gear not retract.
Not in any way an emergency, but a huge management exersize as you are now essentially in a different, lower performance/higher consumption aircraft and need to work out where you can and can't go.
Yes...and as Hapag Lloyd found out the hard way, that you can't go that far.
Add a failed engine enroute, and the problem becomes much more serious.