What are the secret achievements in Day of the Tentacle?
It seems as though the list of achievements for Steam is the same as for the PS4 version, so some cross referencing of this list against your Steam achievements, plus this list of descriptions for all the secret ones should clue you in.
There are a few that are likely candidates, though:
- Chef - requires that you microwave the hamster in the original Maniac Mansion game within DOTT. Just booting the game is (another) achievement, but you actually have to play for a bit in order to get this one.
- Music Appreciation Award - after getting the fake barf, leave the stereo on until (or turn it back on before, accounts vary) returning to yesterday for the final showdown
- Alas, Our Lazy Speechwriters Didn't Cover This - talk to the founding fathers while they're out of the room in Hoagie's portion of the game. They're only gone until you take the quill, so if you grab it right away you might miss this one.
- ESRB Violation - use the fork on the cat while Laverne has both
- Most Annoying Guest Ever - ring the bell at the front desk 100 times
- Master Plumber - flush 50 objects through time, which is more than the required number to win
- Can't Win and Chew Gum at the Same Time - use the chattering teeth on Dead Cousin Ted in the future, before giving them to Washington in the past
There are a ton of secret achievements, though, so running your list against the linked lists is probably your best bet.
Based on the list mentioned by @agent86, I constructed the following lists.
I played the game straight forward without trying to do anything else than win and I got the following achievements. So I consider these as "not really secret", because you must find them to win.
- Number One (DE: Nummer 1)
- Number Two (DE: Nummer 2)
- Flush (DE: Klospülung)
- Yesterday's News (DE: Neues von Gestern)
- Mental patience (DE: Mentale Ausdauer)
- Rap sheet: Trespasser (DE: Strafregister: Eindringling)
- Rap sheet: Yegg (DE: Strafregister: Tresorknacker)
- Rap sheet: Cheat (DE: Strafregister: Mogler)
- Rap sheet: Vandal (DE: Strafregister: Vandale)
- Rap sheet: Thief (DE: Strafregister: Dieb)
- Rap sheet: Kleptomaniac (DE: Strafregister: Kleptomane)
- Rap sheet: Adventure Gamer (DE: Strafregister: Adventure-Experte)
- Moronic drone (DE: Fleißige Aushilfe)
- Only Hoagie can make a tree (DE: Nur Hoagie kann einen Baum erschaffen)
- Only George can destroy a tree (DE: Nur George kann einen Baum zerstören)
- Windsock it to me (DE: Ein Tentakel im Wind)
- Obvious, really... (DE: Echt offensichtlich...)
- Not raised in barn (DE: Keine Säcke vor der Tür)
- Novelty enthusiast (DE: Scherzkeks)
- WWSCD? (DE: Schornsteinfeger)
- Costume quest (DE: Ist schon Fasching)
- Chateau Eau Neaux (DE: Braumeister)
- It's a honor just to compete (DE: Dabeisein ist alles)
- It's a honor just to compete but winning is better (DE: Dabeisein ist alles, aber Gewinnen ist besser)
- Behind every great man is a quality roadie (DE: Hinter jedem großen Mann steht ein toller Roadie)
- That should have worked (DE: Das hätte funktionieren sollen)
- Coffee achiever (DE: Verdammt guter Kaffee)
- Of course it would still weigh the same (DE: Natürlich wäre er noch gleich schwer)
- Don't try this at home (DE: Bitte nicht nachmachen)
- Oh right, I'm playing cartoon! (DE: Stimmt ja, ich bin ein Trickfilm)
- You suck (DE: Supersauger)
- Dialog puzzles are hard (DE: Dialogrätsel sind schwierig)
- Dude, it's like cannibalism (DE: Heißhunger)
This leaves me with 25 remaining achievements, from which only one was mentioned in advance:
- OCD (DE: Sammelwut)
And 24 that I consider as secret:
- Games History Major (DE: Abschluss in Videospielgeschichte)
- Olive Branch (DE: Friedensangebot)
- ... Take on the Words! (DE: Wortschwall)
- We're all brothers (DE: Wir sind alle Brüder)
- That only works with alligators (DE: Das funktioniert nur mit Alligatoren)
- And then the universe explodes (DE: Und dann explodiert das Universum)
- Energy Conscious (DE: Energiebewusst)
- Are you a mummy? (DE: Bist Du meine Mumie)
- Voiding the warranty (DE: Erloschener Garantieanspruch)
- Rap sheet: fraud (DE: Strafregister: Betrüger)
- And then the universe implodes (DE: Und dann implodiert das Universum)
- Now you're pushing it (DE: Das muss nun wirklich nicht sein)
- You call that a hint (DE: Das soll ein Hinweis sein?)
- That joke never gets old (DE: Der Witz wird nie alt)
- Missing Pink (DE: Ich hasse Pink)
- Rap sheet: Forger (DE: Strafregister: Fälscher)
- I'm sure he'd never notice (DE: Das würde er nie bemerken)
- Can't win and chew gum at the same time (DE: Entweder Kaugummi kauen oder gewinnen)
- Master Plumber (DE: Klemptnermeister)
- Most annoying guest ever (DE: Der nervigste Gast)
- ESRB violation (DE: Problem mit der USK)
- Alas, our lazy speechwriters didn't cover this (DE: Tja, unsere faulen Schreiberlinge haben da was übersehen)
- Music appreciation award (DE: Musikliebhaber)
- Chef (DE: Koch)
Once I know how to unlock them, I hopefully remind myself of posting the solution as a spoiler.