When someone tells you to have fun, they usually want you to have the best experience possible.
Respond to “Have fun” with an expression of gratitude, whether the person meant it seriously or not. Because the expression “Have fun” is mainly used to put someone in a good mood.
Whether the person is a complete stranger or a loved one, acknowledging their good wishes helps to build rapport.
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Sometimes a person may tell you to have fun when they know that an aspect of the upcoming experience may be intimidating for you. For example, if you’re going to take a flight and you’re not an avid traveler, they may say have fun in order to help calm your fears.
Their aim is to put you in a more positive state. So, while you might not immediately feel confident about what’s ahead, you can try to align your mood and expectations with their good wishes.
In that situation, you might not be able to muster the emotional strength to say thanks or be totally positive about what you expect to take place. However, you can still acknowledge that you’re hoping for the best.
You could say:
“I hope so.”
“I’m hoping it will go well!”
“Let’s hope that all goes according to plan.”
Express Thanks Nonverbally
When someone says have fun, they may do so when you’re already getting ready to leave. You might be walking through a door or might be at some distance away from them.
In that situation, you might not always want to verbalize your thanks because of the distance. In addition, your mind may already be on the next thing that you have to do. However, you can quickly acknowledge their good wishes.
A nonverbal sign of thanks is often sufficient. For example, you could turn to them and give them a quick thumbs up. You could also smile and wave. You could also nod in acknowledgment of their good wishes.
If the person is a close friend or family member and the occasion they’re giving good wishes for is something important to you, you could say thanks by using an even more meaningful gesture. For example, you could give them a quick hug and a smile as your way of saying thanks for their good wishes.
Confirm That You Will
If you have high expectations of the experience that you’re about to have, you can speak with confidence about it. Share your confidence and enthusiasm with the person who has wished you well.
Let them know just how much you’re looking forward to being there. They probably already know but this is yet another opportunity for you to confirm that you’re expecting the best.
You could say:
“I sure will. Thank you.”
“I expect to. Thanks so much.”
“Thank you. I will.”
“Thanks a lot. I definitely plan to.”
Recognize Consistent Support
The person who is wishing you well might be someone who is always there for you. There are several people who might fill such a role in your life, as you do the same in their lives.
For example, you may have a friend who is always supportive of you whenever you’re going to try something new. Similarly, you could have a colleague at work who always has your back whenever you need to do a presentation or make a report. It helps to recognize the support that these individuals always give.
Showing your appreciation verbally or otherwise lets them know that they’re valued.
You could say:
“Thanks, Sarah. I’m glad to have your consistent support.”
“Thanks, John I appreciate the fact that you always have something positive to say whenever I’m headed off to these meetings.”
“Thanks, son. I appreciate the positive feedback and the words of encouragement that you always offer.”
People who consistently have something positive to say whenever you’re taking on a challenge are to be valued. 
Not everyone will be there for you whenever you are celebrating but there will be people who are happy for you whenever things are going well and are willing to help whenever you face a challenge.
Value these people and let them know that they’re important to you.
Ask for Help
Sometimes a person may tell you to have fun when they know that you’re about to face a challenge. For example, if you’re about to give a report and will be facing a number of tough questions, someone may tell you to have fun in order to encourage you to relax.
In that situation, it’s important to follow their advice and try to put yourself at ease. However, you may also need help from them in order to do that.
For example, you might need files from them or you may even need to do a victory dance before you take on the challenge.
Don’t be afraid to involve a friend or someone else who is supportive of you in a ritual that will help you to have fun.
Be willing to ask for help if that person seems to want you to do well and enjoy your experiences. They may be the one who is willing to go the extra mile with you. In time, ensure that you also look out for them when they’re facing their own challenges and happy times.
Wish Them the Same
Sometimes when a person wishes good for you, it may be appropriate to wish the same for them.
If you both are about to enjoy new experiences and a person wishes you a pleasant one, you can do the same for them.
You could say:
“I hope you have fun as well.”
“Let’s both do this.”
Sharing happiness puts you both in a good mood. It’s always good to bless someone even as they’re blessing you. It adds to your happiness.
Some people may say have fun as a way of telling you to have a good day. And don’t forget the kids. You should do the same with them. When you’re parting ways also tell them to have fun and wish them well.