A picnic is a fun-filled leisure activity which is not stressful or unpleasant (unless, of course, something really unpleasant does occur!).
The everyday English expression No picnic implies the opposite – that the activity is not at all pleasant and could be difficult or extra-stressful.
1. After staying independently from his parents since many years, Ahmed found it awkward to dine with them when he was in town. There was hardly any conversation during the meal and it was definitely not a picnic.
2. All the students unanimously agreed that the CAT exam was no picnic!
3. The scouts realized that the training camp involved a lot of physical activity and was no picnic.