I reply with genki desu routinely, but wanted to use a variety of different responses to answer this question. I want to say I'm fine, ok, well, happy, and other different ways to answer this question. I want to use various positive responses.
there are a lot of possible generally positive responses to "ogenki desu ka":
the standard "(hai,) genki desu." meaning "I'm fine/I'm energetic."
"ma ma desu" meaning "I'm so-so."
"kekko ii desu yo." meaning "I'm pretty good."
"ii kibun desu." meaning "I'm feeling good."
"saikou (ni ii kibun desu)" meaning "I'm feeling awesome!"
there are variations on these, and the list is hardly exhaustive, so I'm sure other people will chime in with other examples.
The issue with these other ways of responding is that they are usually said with the intention of getting the other person to ask you what's going on, in other words, to start a conversation about what is happening in your life.
Just like with English speakers, where the set phrase is "How are you?" "I'm fine." is designed as a casual greeting that shows you're neither ignoring or being ignored, "O genki desu ka" "genki desu" is just how a Japanese person greets and gets on with their day, minimal fuss. Any other replies will draw people into a conversation, so if that is NOT your goal, you might want to think about whether you really want to change up the greeting or not.