Save values

You can directly change save values of your save.ini to change the progress that you have in the game. I would recommend that you play through the game once to unlock everything, the game is worth it, but if you somehow lose any item or character you earned this will show you how to get them back easily.

You could alternatively use a save editor developed by one of the members of the Risk of Rain forums here. (But as so far, it only unlocks characters)

Monsters
All monster values are in order of appearance in your monster log, apart from the order in achievements and directly correspond to the monsters place in the log. Ex: mons1=1 in the save.ini in the second section makes the lemurian log appear, as it is the first entry.

Achievements
When you save the following commands in the [Achievement] section on its own line, it has certain effects on your game. To do this, make a new line stating "achievementX=2", where X is the number of a given achievement you want to unlock, preferably under other completed achievements (or their progress in case you unlocked none, if not even that, achievements are on the top of the file).

Character Unlocks:

6 = Bandit

14 = Enforcer

17 = Han-D

20 = Huntress

23 = Engineer

26 = Miner

29 = Sniper

32 = Acrid

47 = Mercenary

Item Unlocks:

0 = Gasoline Item

1 = Unstable Watch

2 = Soldier's Syringe

3 = Lost Doll

4 = The Back-Up

5 = Paul's Goat Hoof

7 =  54 Leaf Clover

8 = The Ol' Lopper

9 = Pillaged Gold

10 = The Hit List

11 = Fireman's Boots

12 = Photon Jetpack

13 = Hyper-Threader

15 = Spikestrip

16 =  Prescription

18 = Arms Race

19 = Shield Generator

21 = Hermit's Scarf

22 = Predatory Instincts

24 = Sticky Bomb

25 = Concussion Grenade

27 = Panic Mine

28 = Shattering Justice

30 = Crudely Drawn Buddy

31 = Telescopic Sight

33 = Toxic Worm

34 =  Massive Leech

35 = Dio's Friend

36 = Dead Man's Foot

37 = Alien Head

38 = Bitter Root

39 = Captain's Brooch

40 = Glowing Meteorite

41 = Filial Imprint

42 = Golden Gun

43 = Wicked Ring

44 = Harvester's Scythe

45 = Snake Eyes

46 = Ancient Scepter

48 = Chargefield Generator

There are 48 achievements in total that you can unlock with this method, characters and items included.