I won't say it is a must, but yes you can check out if you are interested in theme parks. If you go to Fantasy Kingdom, you must check out water kingdom. You can also visit another theme park called Nandan park, it has more space than fantasy kingdom. And if you go to Nandan, you must check out Water world.
I have been to both Fantasy Kingdom and Nandan many times, not because i love rides, because there is not much things to do in Dhaka. And my office organized picnic many times there.
To answer your last question, yes, Fridays are very crowded, better try on saturday.