Hi Milly. I can see how you became defensive. I wouldn't like to come on here and ask a question about my heartfelt desire to help my children have fun and have the best start in life met with a host of derisive snorts from a bunch of seasoned mothers.
And just because you get something from getting out of the house and meeting friends and going to these activities, doesn't mean you are a lonely saddo, or that you don't do other valuable things at home with your daughter.
And it's lovely that you are both enjoying it. You are certainly not doing anything wrong -- although I would be careful in the future (I'll say why in a second)
Structured activities for babies are not my cup of tea. I have enough in my life to be on time for, I don't need adding to it! Besides, I don't get much from meeting other mums just for the sake of their being mums. That said, I can see how some structure, some order and a seeing your baby enjoy something is a real boon to some people. Your (probaby) working one minute, then staring into the chasm of 24 hours a day with no order. It can seem daunting. I certainly found it daunting (my daughter is 2.5 and I'm pregnant again) which is why I'd make a point of going out each day to break it up, change of scence, see new things, etc.
That said, I do think exposing babies to different things is good. From an early age I swam every day (luckily she was born in Singapore so was a water baby from 8 weeks), we go to the zoo/science museum/Gambado/library/toy museum/parks/swimming/other stuff each and every day. I know friends think we are a bit precious. For other reasons too -- I still bfeed and still carry my toddler in a wrap. We all have our pfb-weaknesses!
However:
You baby learns at this stage from you. She needs you and your breasts if you are feeding, and pretty much nothing else. The idea that she is learning about ballet is a little silly. She may seem to enjoy this time but that could be for other reasons. You could be particularly animated (the whole class-ness of it, and enforced jollity tends to produce this), you are giving her undivided one-on-one attention and there are things to look at and hear. But learning is an organic and continual process (she'll learn as much about proprioception and movement trying to reach for your jewellery, tweak your nipple, try to climb stairs when you are not looking than half and hour of tambourine clapping an throwing her in the air. Socialisation? She learns that from you at this stage.
Sometimes a heavy diet of structured classes can kill a bit of the spontaneity that helps children develop. It could be tempting (I'm not for one minute saying that you do this but I know others who do) to think 'I've done my 'fun' time' and not be prepared to ceaselessly play or follow your child's lead at home. You know, get the sheets off the bed and make tents, have baths in the middle of the day to splash the hell out of the bathroom, messy painting, reading, etc.
Beware the law of unintended consequences. If you keep this up, you run a small risk of making your daughter resistant to learning something. I say this having talked to a friend who is a swimming teacher. She comes across a lot a parents who are desperate for their children to swim and pay for expensive lessons and exude 'please son/daughter, swim for me, it will please me'. In many cases the child simply resists and loses interest and will only learn at his or her own pace. Children are perceptive about emotions and can read us without us speaking. If you give away that you 'want' her to learn, she could turn away later in toddlerhood.
I say this as I started teaching my daughter to read (sight method) when she was 18 months. She'd been picking out some words (baby, zoo, etc) by recognition for some months and so I thought I was following her lead. However by a few months later although she'd learn many words I could tell she was losing interest a bit so I stopped. Learning and loving reading is too important to 'force' and I wouldn't want it to be related to 'pleasing me'.
A 5 month old will go anywhere and do anything. A 9+ monther may begin to balk and toddler even more. So what she enjoys now she may not. My 1 year old suddenly took against Rhyme Time -- hated the yakkety mums and wailing babies and just stared, bamboozled, the whole time.
Consider the amount of travel time to and fro. With a frequently napping 5 month old, not so much an issue, but many toddlers don't often enjoy car journeys/public transport.
Regarding signing lessons (something I too considered). I wouldn't, unless there is a speech delay issue. If you want to bring on her speech (I did this with my daughter and she spoke in 3-4 word sentences at 13 months, don't know if it was solely down to this though and simply her) I followed the Babytalk programme from Dr Sally Ward (paperback book)
What you are doing is expensive--by any measure. How about saving that money for something arguably more worthwhile, such as private education, supplementary education, such as Kumon, piano lessons when she's 4, riding lessons, etc?
Play constantly, be silly (I really recommend Playful Parenting by Lawrence Cohen for ideas), whack on the Schubert or Mozart (babies seem to love Mozart), have a read of Babytalk, maybe play on the piano a bit or buy a cheap keyboard, get some musical instruments, messy painting stuff and most of all enjoy her. I miss that 5 month old stage!