Alright, I'll assume that most of you already know about the Matilda gate trick, but just in case:

Well, it turns out that, using that same trick, you can access a handful of other places early on as well. Normally, if you go into Greenhill early (either by using the Matilda trick and then traversing through the Forest Path leading to Greenhill, or just going through the Muse-Greenhill border), you're limited to Greenhill and Forest Village. The Kobold District of Two River, to the south of Greenhill, is blocked by a kobold, who kicks you out as soon as you try to enter. However, I just recently did some experimenting, and you actually can get there not long later.

Here's how it works:
(For a simplified version of this, scroll down to the bottom of the post.)

1.) Wait until Muse gets invaded. After you escape, before going to Coronet, head up to the Muse-Matilda border and go through the gate. (Warning: Riou and Nanami will be your only party members at this point, so it is recommended that you level up in Matilda using the same trick beforehand. Picking up Humphrey and Futch along the way wouldn't hurt, either, even if you only do so temporarily, rather than completing their quest.)

2.) Take the Forest Path into the Greenhill area. (The enemies here will be more powerful than before, but they shouldn't be a problem if you were fine in Matilda and the Forest Path.) Once you get to the Greenhill area, you cannot turn back, so make sure you want to be going through all of this.

3.) Head down to the Kobold District of Two River. Unlike before, you will be allowed in! This immediately opens up access to some great items at their shops, as well as more items at the other shops in Two River. (If you're really lucky, you can ever get some rare finds, like Silver Hats and Warrior/Wizard Runes.) You will likely have your first encounter with Bob here, but it's still too early to recruit him. Unfortunately, due to not having Gengen in your party, you cannot recruit Gabocha just yet, either. However, Hans, the armorer, will already be available to recruit, at the inn in the Human District! (Regardless of not having your own castle setup yet, you can still invite him there. It's kind of humorous seeing him talk about "_ Castle," since you haven't named it yet.)

4.) Doing some more traveling this way, you can also access Kobold Village and Tigermouth Village to the south and southwest (respectively). If you have any Crystal Balls, feel free to sell them at Kobold Village, and you might even be lucky enough to find some Gold Emblems (+10 DEF, +10 MDEF) in the rare finds at the item shop! (Not to mention the free Exertion Rune in the pot outside the mayor's house.) There are also some recipes and seeds and such to be found/bought at these locations.
To the east is Laskewest, where you'll already be able to recruit Tetsu (if you use Fried Tacos, bought in the Kobold District of Two River, to become Toasty) and Taki. However, Hix and Tengaar do not appear yet. Shilo is also unavailable at the moment. Load up on whatever items you'd like while you're here (Don't forget the Kindness Rune in the basket of veggies at the dock!), and then head out on a boat.

5.) Now that you're on the boat, head to Kuskus and continue reaping whatever benefits lie down that way. (North Window is not available, of course, since it's not your castle yet. Coronet will also be unavailable, I believe. [Haven't confirmed it, myself.]) In Kuskus, you'll find more recipes and other items, as well as the blacksmith, to sharpen your weapons to level 6. From Kuskus, you can travel east to Radat Town, and you can go south to South Window.
If you go to Radat, be careful at the bridge. It's still blocked by that group of men, and if you get too close, you'll be forced out of the town, which will lead you to the area of the Mercenary Fort. (That means you'll have to walk all around back up to Muse, and do everything over again if you want to explore more in the south.) You won't be able to recruit any characters here yet (except maybe Lebrante, if you happen to be carrying a Celadon Urn), but at the item shop, you might be lucky enough to find a Sound Set, Magic Robe (+22 DEF, as opposed to the crappy +7 DEF from a Guard Robe), and/or Speed Ring (+10 DEF, +15 SPD).
If you go to South Window, don't worry about progressing the story unnecessarily. You can come here and wander all you'd like. This will provide you with early-ish access to Lightning and Wind Runes, some other items at the item and armor shops, and a free Friendship Rune (in one of the potted bonsais on the far east of town). Nobody here can be recruited yet, unfortunately.

6.) Whenever you're ready to continue the story, you'll have to go through Radat and walk back up to Muse and Coronet.

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Simplified Version:
1.) Immediately after Muse is invaded, use the gate trick to get into the Matilda domain.
2.) From Matilda, take the Forest Path to the Greenhill area. Once here, you will not be able to turn back.
3.) Two River, to the south of Greenhill, will now be open, so go there and do as you please.
4.) After Two River, you can access Kobold Village to the south, Tigermouth Village to the southwest, and Lakewest to the east. You'll eventually have to go to Lakewest to continue.
5.) Taking a boat from Lakewest, you're limited to Kuskus to the southeast. From there, you may go to South Window and Radat.
6.) To get back to Coronet to continue the story, you'll have to go through Radat, and walk back up through the Muse area.

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I'm not sure if any of you knew about this beforehand, but I just discovered it (though I get the feeling I also did it last time I played...), so I figured I'd share it. A handful of the things you're allowed to do this way are available only shortly after going through Coronet anyway, but the things in the Two River area are available much earlier than usual. I'd say some of those things alone are worth the trip, personally.

Anyway, if any of this turns out to be incorrect, or if you find anything to add, feel free to say whatever it is.

Haha, I never knew that, though I never even tried myself. I don't like walking that much on the world map O_o.

I'm always too into the story to do this, but it's a neat trick.