A Word from Remus
Once you're out on the world map (assuming you're starting from Oleg's Trading Post) travel south, then when you reach a fork, turn east. You won't get far before you're delayed by a random encounter, where you'll meet with an odd old man named Remus. Listen to him babble cryptically for a bit, then when you go to leave he'll mention that your rival, Tartuccio, is in an Ancient Tomb looking for an artifact. This marks the Ancient Tomb on your map and updates the quest A Bitter Rival. If you're rude to him, you'll get the same intel, albeit worded a bit more vaguely, and if you choose to attack Remus, you'll get credit for performing a [Chaotic Evil] action, but there's otherwise no reward for doing so.
Discovering Two-Rivers Field
Use the area transition to leave the locale where you ran into Remus, and when you're back on the world map be sure to save your game. If you read the world map exploration tips earlier, you may recall that one of them warned you of hidden areas. Your destination, Two River's Field, is one such location, and an unwary traveler could just walk by it none the wiser. Let's make sure that's not you, eh?
Continue east from the encounter with Remus, only to find the path bending south. When you reach an empty node, continue south and you should be prompted to turn west towards Fangberry Cave. Ignore it and continue south to another empty node, then head southeast along a path that ultimately terminates at the Shrike River. If you did not discover the Two Rivers Field area to the northeast of this path as you traveled along it, reload the game and try again. It may take several tries until you do. You'll find it marked by a black circle on the map, and if you have the "Show location's names" option on, there will be no doubt as to what the area is.
Travel back up the path if you need to, and when you're adjacent to the now-discovered Two-Rivers Field area you should get an arrow prompt to travel northeast to the aforementioned locale.
Note: The RNG can be pretty brutal when it comes to discovering hidden areas on the map, and if one just refuses to appear for you, it may be better to just write it off for now and try again later. We can confirm, however, that saving at an adjacent node before traveling along the path the hidden area is located off can cause it to appear, it just may take a few tries.
Note: Acquiring new characters (and traveling with them in your party) and leveling up can cause previously stubborn hidden map areas to appear on subsequent visits. Be sure to try again later when you meet the aforementioned conditions.
Exploring Two-Rivers Field
Do so, and when you arrive in the area from the world map, make your way west to the southwestern corner of the map to find a Boggard. Although sturdy and fairly strong, Boggards have low AC and are generally not much of a threat. Their worst attack is their Terrifying Croak, which will cause affected characters to become Shaken (-2 penalty to attack rolls, saving throws, skill checks and ability checks - but they can still fight, so it's nowhere near as bad as being frightened or panicked) should they fail at a Will 13 check. Once the Boggard is dead, make your way north along the western edge of the map to find a trap [Trickery 17] which can be disarmed so you can reach a pile of rocks hiding some loot [Perception 15], including a bunch of trinkets fit for selling, and a Scroll of Inflict Moderate Wounds.
Next make your way to the eastern end of the map, where you'll find another Boggard roaming about. Kill it and head north along the eastern edge of the map to find a Wolf mourning over the corpse of its companion. Reunite the two in death, then be wary of several traps [Trickery 17] that are placed to the north, between the Wolf and a campsite.
Search the campsite and pass a [Perception 15] check to find a sack behind the tent. Loot the sack for various food-stuffs, then plunder the more obvious box that yields a Shortspear +1, an Ancient Rostland Coin and a Scroll of Hurricane Bow.
Note: Keep any Ancient Rostland Coins you find on your travels. As the item's description suggests, an "antiquarian" will indeed pay dearly for them - more than a merchant like Oleg will, in fact. This holds true for any item with a similar description. Don't worry, they'll be pointed out as you find them.
Make your way back to the bottom-center of the map and head north to find two Wolves fighting a Boggard - or more likely the aftermath of that fight. Kill whatever is still alive, then continue north and west to find another trio of Wolves in the northwestern corner of the map, should you favor a bit of random carnage.
With that, you're done with the Two-Rivers Field. That wasn't so bad, was it? Most optional map areas are pretty small, one-encounter affairs that allow you to score some loot. If you feel up to it, there are more encounters you can complete before returning to Oleg's Trading Post to rest and recuperate, random encounters permitting, of course.
If you're partaking in the first expedition suggested at the end of your first stay in Oleg's Trading Post, continue with the next area, Nettle's Crossing.