Majora's Mask Part 1: Dekus, Gorons and massive, bodiless singing giants, oh my!

South Clock Town, dominated by the aforementioned
Clock Tower.

After the Happy Mask Salesman's little temper tantrum and subsequent quest-imparting, you end up in the main hub of Majora's Mask - Clock Town, like I previously mentioned. The only really important stuff that happens here is sidequest-related, so largely it serves only as somewhere to start at after restarting time.
The first thing you'll notice is that, as a city, it's much larger and more vibrant than anything in Ocarina of Time - there are many more shops and independent buildings here, loads of townspeople going about their business, and a much more organic atmosphere. This is largely a result of the increased RAM allowance now that the Expansion Pak has been installed, and it really changes the entire feel of the game.

The Southern Swamp

You may also notice that many of the townspeople are on their own schedules, independent from yours - for example, the innkeeper, Anju (who uses the same sprite as the Cucco lady from Kakariko village in the previous game) moves around the city and the inn depending on which day it is and what time of day it is. Many of the sidequests require you to find and speak to people at certain times of day (or night,) so it adds a lot of interest to the gameplay. Even the weather changes from day-to-day - for example, the first day is sunny, the second day it rains, and the last day is punctuated by occasional earthquakes. 

Your first order of business, like Tatl mentioned, is to check out the Southern Swamp, accessed by leaving town from the south gate. (Logical.) Now that the Mask Salesman has changed you back into a human, your Deku scrub form is available to you by means of the Deku Mask - you can put it on and take it off almost anytime. There are around two dozen masks in the game, some optional, some not, and all with their own unique powers. The Deku form, for example, lets you hover for a short time and blow magical spit bubbles (I know, I know) which can come in handy in a variety of situations.

The Deku Palace Gardens. Make sure you have a
stress ball nearby.

There are only four temples in the game, so to create length and interest, it takes the completion of several quests in order to access each one. After you complete the right quests, Link will learn an Ocarina song which will allow you to enter the temple, and most of the time he'll gain a new mask which will allow him to change forms or gain a new power. Some of the quests can be tedious, mostly because they often require a lot of backtracking to get items or masks that you didn't know you needed. 
 
In order to get into the first temple, for example, Link saves the witch Koume, who has been knocked out by Skullkid, with one of her sister Kotake's potions. Then, some adorable white monkeys tell Link that their brother has been imprisoned by the Deku King under suspicion of kidnapping the Deku Princess. Link, being the great guy that he is, goes to save the monkey, which involves traversing a couple of Hyrule castle-style mazes in Deku form. If you get spotted by the guards, you have to start over. I don't think I need to tell you how old this gets.

Woodfall Temple as Deku Link. You can see a Deku Flower
in the top left, which lets Deku Link hover.

After you do all this, you find the monkey, who tells you that the princess has actually been captured by a monster in the Woodfall temple and that you should probably go save her. (Go figure.) He teaches you a song that will let you open the temple, and then you get caught by the guards and kicked out. Stupid guards.

The temples in Majora's Mask can be hard, although much of the difficulty comes from non-linear layouts rather than head-scratching puzzles. The Woodfall temple is straightforward, though, and has incredibly unique and well-designed rooms and music. A lot of attention was paid to location-appropriate enemies and obstacles, which contributes to an immersive (if sometimes really, really frustrating) experience.

I'm going try my best to ignore the uncomfortably racist
pseudo-African look of Odolwa in favor of gently
reminding my audience about the importance of sensitivity.

All the temples follow standard Zelda format - there are a few rooms in which you have to defeat several enemies to open new doors, all temples have at least one mini-boss, and you have to collect the boss key to proceed to the end. All the temples also feature small keys, maps, and compasses to help you on your way.

There are new, species-specific obstacles as well - Deku Link can use Deku Flowers to hover or hide from enemies, Goron Link can smash large switches and push blocks that regular Link can't, and Zora Link can swim and breathe underwater.
The bosses in Majora's Mask range from hilariously easy to stupidly difficult, but all four of them are totally unique experiences. The first boss, Odolwa, is the only one which you can fight without the aid of at least one mask, and the arena in which you fight the bosses range in size and style. (You have to become a giant to fight one boss, which is awesome in a Godzilla kind of way.)

Link, what did I tell you about cough syrup?

After you complete Woodfall temple, you are teleported to a bizarre bubble world and come face-to-face with what I can only describe as a massive head with long, spindly arms and a decidedly Roald Dahl-esque look. It (he? She? Zhe?) teaches you a song and instructs Link to "call us." I wish I could tell you that's the last time you see these freaks, but there are four - one for each temple - and they ultimately save your ass from being crushed by a giant, angry moon. It's all par for the course, honestly.

You're given the item "Odalwa's Remains," rescue the Deku Princess by stuffing her into a tiny glass bottle (I don't even know) and then free the monkey from his execution at the hands of the Deku King. The swamp, which previously had poisoned water, is all healthy again and whatnot, and you saved the day! Isn't that great?

And I thought the Wind Fish was bizarre.

Except not really. This is where the dark side of Majora's Mask comes in - once you return to the first day in order to save, everything goes back to the way it was. Nobody has saved the princess yet, the monkey is still about to be executed, Koume is still dying alone in the forest, and the swamp is still full of poison. The only things that you retain are the songs you've learned, the key items you've gained, and Odolwa's Remains. You've saved the giant which was trapped inside Odolwa's Remains, but nothing else has been resolved.

But, you only have three days to save the world, so you can't help them. Instead, you're forced to move on to the next area if you want to get all the giants to help you stop Skullkid.

With that happy thought, stay tuned for the next section of Majora's Mask - Snowhead Temple.