21 years is quite a long horizon so I think it makes sense to invest it in something with a bit more risk (and so usually better return over the long run) than a simple savings account. Historically, there were long periods where savings rates were below the rate of inflation so in reality, although you may end up with a bigger number after 21 years, it may actually have less purchasing power than the €1000 today.
Historically, stock market returns tend to outperform equivalent savings returns over the long run. This does not guarantee shares will out-perform savings over your 21 year stint but you could say that it is a more probable outcome
You could invest it in a fund. I recommend a passive fund over an actively managed fund as the commission rates are usually considerably less and tend to outperform the actively managed ones anyway. You should shop around for the cheapest fees. This includes fess to enter and exit the fund as well and most importantly the funda management fee. I think Rabobank have a number of funds avaialable with mnagement fees around 1% but you may do better
Another option is to buy a some shares yourself. This would negate the ongoing management fee but I believe some places have ongoing account maintenance fees so you need to shop around. Also, brokers tend to charge a large transaction fee but shop around and over 21 years this should work out quite small per year. You could reduce the risk by buying a number of different shares.across a number of different sectors.
Maybe some others have a few ideas of alternative investments