1. We made side dishes with the ingredients we had.
2. We cooked side dishes with the ingredients we had.
(Are both OK?)
3. What is the difference
between 'bonfire' and 'campfire'?
4. We did the clean-up activity at the Han River.
5. We did the clean-up activities at the Han River.
(Do we have to use the singular form or the plural form?)
Yes. The first two sentences are correct.
A campfire is usually a small, controlled fire for cooking or warmth. A bonfire is much larger and is used to celebrate something.
Both 4 and 5 are right. Whether you use the singular or plural depends upon whether you have just one or more than one activity. However, usually clean-up activities at a river involves several activities.