I voted cello becuase I will be 47 soon and was just wondering if it is too late for me to learn the cello.
It’s fine for fitness and developing awareness and control of your body. It’s generally not practical for street-type self defense until you’re rather advanced. While it can be vigorous, it usually doesn’t lend itself to injuries from training.
Aikido has no attack techniques. Practice/training is done against various stylized attacks; what you learn is how to deflect and control the assailant. Since it’s a body of defense techniques, there are no Aikido competitions.
There are different branches of Aikido. Aikikai is directly descended from the founder of the art and is headed by his family. Some branches received the founder’s blessing, others did not.
Go to a dojo and ask to observe and/or attend an introductory session. See if it suits you.
I’m 45 now and took up disc golf 3 years ago. I love it.
It’s cheap, gets me outdoors, and can be done with no prep. Play by yourself or with others. I learned by watching a lot of how-to youtube videos.
You can find a list of all the courses in your area using sites like dgcoursereview.com or pdga.com.
While the sport seems to be heavily dominated by 20 somethings there are quite a few guys I’ve met on the course in their 40s & 50s.
Tory scum 
My primary hobby (SCA www.sca.org) has brought me to learning several art forms, basic lampworking, pewter casting, and leatherworking. All are inexpensive and easy to get into. With a little work you can create some great things for very little money.
Seeing that you enjoy music and are looking to get in shape, I would suggest you try social dancing. It is a lot more common than you might think, it’s inexpensive (some bars will offer free lessons, although imho you are better off taking classes at a studio - usually about $15 an hour) and its fun.