You should say:

　　when and where the incident you remember happened

　　who you were with

　　what happened exactly

　　and explain why it is a happy memory.

　　雅思口语part2高分话题范文参考：a happy childhood memory

　　I grew up very far from most of my family so I only saw them once a year. Every summer I would go to stay with them, while my parents continued working. Although I missed my parents, Iused to really enjoy spending time with my grandparents, cousins, aunts and uncles. I spent all summer there, nearly two whole months, so from time to time my cousins and I would get bored. We would ask my grandparents if they had any ideas for interesting things we could do. Sometimes they suggested going for a bike ride, sometimes into town to do some shopping. At other times, they showed us a new game to play. Then we were happy again.

　　One day - it must be about 12 years ago (doesn't time fly!) - all of us got really, really bored and we kept complaining to my grandparents. They were tearing their hair out, trying to think up ideas of where we could go and what we could do. Suddenly, my granddad came up with the idea of going to a new water park that had opened that summer. I hadn't heard about it but my cousins had, and they told me all about it. It was a park with vast numbers of different pools, some inside, some outside. There were water slides as well. And, on top of there wasnot one, but two play grounds with swings, a merry-go-round, see-saws. We were so excited.

　　We set off and on the way we were all singing songs and laughing. We couldn't wait to get there. When we arrived,my cousins and I ran into the park and changed into our swimming costumes. Then we went looking for the most exciting looking pool, and we found it, one with brightly coloured tiles and slides, we jumped straight in. We played all day in the park and had a lovely lunch, sat on some benches in the sunshine.

　　My granddad loved a pool that was filled with spa water. It was dark brown and stank of rotten eggs. I didn't wantto go in but he eventually convinced me. I'm pleased he did because thewater was really warm. I'd never swam in water that warm. I didn't want to get out, despite the terrible smell.

　　I consider it such a happy memory because we enjoyed ourselves so much and I remember so vividly how I felt that day. But there's more to it than just that. When I look back now, I understand how caring my grandparents were and how much they wanted us all to be happy. They wouldhave done anything to help us have a good time. Iappreciate that more now that I'm older andhave children of my own. I hope we thanked them; I can't remember.But, anyway, they were content, I'm sure, to just watch us have an amazing time, playing and laughing in the water

　　So, as you can see, it was a wonderful day and is one of my favourite childhood memories.