The secret to having some fun as a speaker is knowing that you are giving some joy to the audience. It does not mean that you are stand up comedian whom everyone is laughing at. It will be fun and enjoyable because i will be so comfortable performing that i can think in the moment, react in real time, make fun of yourself in case of a blunder, and it will end up being fun to them.
I also know that i am giving them valuable information that will prove to be useful in their lives and careers which is why they value and respect me and show appreciation. This makes it a fun experience for me. Everytime i do a training, people come up to me and say thanks or how my words have impacted their lives and that makes the experience so pleasurable.
So when you are speaking, whether it is on a Monday morning conference or in an office setting, give them information that they can look forward to hearing from you and gain something of great value out of it.