Why didn't Hogwarts come up with a better way for spectators to watch the other two events?

tl;dr: Since the story is viewed from Harry's point of view, we follow him as a participator and don't get shown (in either book or movie) how the spectators, for whom stands were built at each location, observe the proceedings, but it is presumably via magical means.

This is going to be a tough one:

The Tournament, called the Triwizard is held (for the first time in over a century) at Hogwarts during the protagonist's (Harry's) fourth school year.

It is called Triwizard because there are three trials, and because three (a trio of) schools compete in it. Each school sends one contestant, to be selected by the eponomous Goblet of Fire. Due to machinations by the bad guys Harry gets selected as competitor, called champions as well.

As to the tasks:

During the second one, it is not students that are tied to the lake, but people that are personally close to the champions. Since it is happening at a school the hostages turn out to be students, yes, three of them are even students from Hogwarts Harry knows and cares about: his best friends Ron (his own hostage) and Hermione and the girl he has a crush on (Cho). But the fourth one is Fleur's little sister Gabrielle (why she is there the whole time (she arrived with the rest of the Beauxbatons girls) eludes me, since she should be 8 years old). And only Harry uses gillyweed, the others use other methods of underwater operation (shark transfiguration, bubblehead charm); it's at the champion's discretion.

During the third one, the cup in the middle that teleported them (Harry and Cedric) away was not supposed to do that, again due to machinations by the bad guys. It was not supposed to be a portkey at all.

In both the book and the movie (the latter being the topic of the question) we follow Harry's story. We actually don't see (in either medium, because Harry doesn't watch the tournament, he is a part of it), how the spectators (for whom they built stands after all) observe the event.

It could very well be via magical means. The book version might contradict this, because Professor Dumbledore converses with the chieftainess of the merpeople in the lake after the task, but then again, that could be just for clarification as to the reason why Harry didn't leave with Ron once he got there, but waited for all hostages to be rescued (In the movie the merpeople are actively hostile and not just playfully so, as they are in the book, so Dumbledore doesn't talk to her).