I don't think it sounds "too strong" to say, "Yes, I do" or "No, I don't." However, if we are having a conversation, I might hope that you would add some more information. So, if I ask if you like playing badminton, perhaps you could say, "Yes, I do. I play almost every weekend." If I ask you if you like swimming, you might say, "No, I don't. I never took swim lessons." Of course, it is also possible to answer in other ways...and using a variety will help keep a conversation interesting. "Yes, it's one of my favorite things to do with my friends." "No, not very much." "I *love* playing badminton." Your friend's ideas are also good. So, saying, "Absolutely -- I like swimming very much." will sound good. But I don't think it is necessary to always use specific words instead of simpler words like "yes" or "no."

Hi Fan! Im a native English speaker and yes often times just saying No I don't can be harsh but it is not grammatically incorrect. For example, if i ask: Do you like spicy food? You can say, No, I don't but you can also say, "Not really" or "Not particularly" or even "It's not my favorite" which can imply no. Hope this helps!