Dance and gymnastics are enough for now.
Per piano, my son at 5 years old, started piano lessons. Why? Because HE wanted to. He likes it. And practices without even us telling him to.
Guitar: you'd have to get a kid sized guitar.
OR... try a Ukulele. Its smaller. It is a very great instrument. And many pop musicians even play it too. Very versatile.
*Adding this: many young kids in Hawaii, grow up learning the Ukulele. They even teach it in schools.
My kids have many interests... but we cannot possibly, sign them up for EVERYTHING. Because, they are in grade school (homework is important and having the time for that), and the costs, and the sheer amount of energy we have or not, to take them to all of those "lessons."
And we clearly, explain that to our kids.
So then, they wisely "choose" what they want to learn. For now.
As well as us discussing it with them. We don't just sign them up for everything they think of. A kid also has to learn, choices and why. But at a later time, they can take other lessons.
My kids take Karate, piano, and tennis.
They want to. We can manage that for now.
My kids are now 6 and 9.
My friend's son takes guitar. He started when he was 7.