Converting it to fiat and investing in real life business is the best thing one can do, as real life business will give him regular income and profits and it don't have risk of disappearing like bitcoin.

I will buy a car.

I wouldn't say I won't be happy but I doubt 10 BTC will be that much of a changer for how you spend money or the like, I would probably keep it as bitcoins and try to earn some profit on it in the market.

 I am going to invest the half of that amount and I am going to keep holding the remaining ones. But I don't know if someone is really going to do that. Going to give 10 bitcoin for free. I am going to thank him too whoever is he. So, that next time if he appreciates my thanksgiving then he is going to consider me again.

If  i would be given  a 10btc amount then for sure  i would  keep the 7 btc in my wallet for long term investments  and 2 btc for  trading investment then 1 btc for cashout buying  my needs for everyday living  that would be my plan. I would gain profit  for my 2 btc invested in trading  and i would use it   for everyday living without touching the 7btc kept on my wallet.

i would save it all, spending your bitcoins right now might be really dumb to be honest because in my opinion the price is going to grow a lot

Yeah that is the right thing to do, its good to be safe as 10 btc is really a very big amount and no one wants that they loose it by investing in risky sites.

I think he just made a joke , for me tbh better to trading on altcoin for getting a good return because if i keep hold my bitcoin in wallet , my bitcoin is not really "growing up" only the price of bitcoin will be rising in the future.

Yup if you cannot afford to take risk then its good that you save them for future to earn profits from it, as investing in trading  involves risk of loosing it if your decision goes wrong.

Probably convert it to cash and setup a stock portfolio