My daughter is 3 months old and has started responding to my and my partner's voices. We talk to her in different languages, play soft classical music, show her how to use her hand, and do all other stuff that keep her interested. While not substantiated, we believe that these activities help her learn faster. Are there any other tried and tested methods to help my kid attain a high Intelligence? Specifically, are there any toys and activities that have been scientifically shown to be effective?
I think a loving, healthy home is an excellent foundation. As a special needs teacher, I can tell you that parents might not control how intelligent their child might be, but that a bad influence can and does matter. You can help them achieve their best self. (I am not talking about parents who smoke or do drugs or did drugs while carrying a child -- of course that's a negative influence.)
Read to your child. Even now it is not too early. Be honest with answers. (Make sure you understand the question; 'where did I come from Dad?' could just mean that his friend is from India, and he wants to know where is he from.) Try to limit TV to stuff you can watch as a family. Discuss what you've seen. Listen to music. Show your child that you read and question news and information. Read fiction, non-fiction, history and science. Talk about what you are reading. Do homework as a family. Everyone is reading or writing or helping. I am not suggesting that you do their homework at all. But I hated being shoved away to do homework. Make it into an event.
If they make a mistake on their homework, explain it. Find out what information is not clear to them and explain it if you can. If you do not know -- admit it and then go look it up or ask their teacher for help. Make the child asking you into an opportunity. BUT, let them fail, too. When they do, say "Okay, what went wrong? What can we/you do to make it better or right?"
Get lots of exercise and play or do a sport together. Build your kid up. Encourage them.
Let them know that everyone makes mistakes and that failure is a great stepping stone to success and innovation. When you make a mistake or break something -- admit it! "Oops! I dropped the kitty litter bag. I need to be more careful. I will clean it up. Would you help me?" Give your child responsibilities. Chores are something the entire family does.
Providing a loving and supportive environment will provide the platform for her to become her best. I would not be concerned with her IQ because that is a genetically driven, not environmental. If the parents have a high IQ (>130 Stanford Binet) then there is a good chance normal offspring will be as well. Think of IQ as a light: the parents must guide the aim (guide) the light and focus it to shine brightly.
Children that are play musical instruments successfully are highly correlated with academic success: there is not causality between the two.
Disclaimer: I don't have a source for this (and I'm too lazy to search it out).
I read something a few years ago that boiled down to: "a greater variety of words heard at young ages directly impacts a child's IQ/intelligence".
The context of it was an article discussing single parents who are forced to leave the child in front of the TV vs parents who had the time to interact with the child and thus expose them to far more varied words and concepts. Also, in this case the child was essentially babysat by the TV for HOURS each day. Normal TV watching for short times was outside the scope of what was being discussed.
Now studies are saying that IQ is genetic, but I believe it is also a fact that new paths in the brain are formed as different concepts are introduced, learned and linked to existing concepts and memories. So at the very least the nurture part of brain development can be maximized/optimized (within reason).
So in answer to your question: talk to your child in a very descriptive way such as: "I'm opening the drawer to get the spoon. See the spoon? Its made of metal and is cool to the touch. Now we are pouring cereal into a bowl. Can you hear the bag rustling? Milk comes in big plastic jugs - notice how white the milk looks?" etc.
IQ is mostly genetic and partially environmental. So...
How can you maximize your child's IQ potential?
note: all primary sources are in video descriptions
Breastfeed 18 months if possible. Breastmilk is genetically tailored for your child providing a large range of benefits, formula is not. Physical contact with the mother is also a large part of gaining a sense of security for the child. https://youtu.be/qSJG2QS4mvI
Don't abuse, spank, or use harsh punishments. https://youtu.be/sBm8i96ZGcQ
Teach your child Philosophy. https://youtu.be/8wY1m2MqXj0
Presumably, provide a stable, loving environment and don't leave your baby crying alone in a room all night.
Peaceful Parenting Series: Raising Children Without Aggression: http://www.youtube.com/playlist?list=PLMNj_r5bccUwZY7RCZnS2e5-vjaA7wSNw
Real Time Relationships (PDF download) http://www.fdrurl.com/RTRPDF
Of possible interest: On Truth: The Tyranny of Illusion (PDF download) http://www.fdrurl.com/OTPDF
Of possible interest: The Origins of War in Child Abuse (MP3 Audiobook download) http://cdn.freedomainradio.com/OWCA_Audiobook_64.mp3