"Diable!" is similar to "What an unpleasant surprise!". What word(s) can I use to convey the same meaning? It should be not religious, understood and hopefully short.
A lot of people say Fek! It might be considered slightly more offensive, however, depending on your point of view.
"Diable" and "Ho dio" are the only widespread equivalent to the English phrases of mild surprise. "Ho ve" is often heard but is often said with a more disapointed tone, and "fek" is more abrasive and obscene than "diable" or "ho dio". Despite being religious references, neither of them are semantically religious, as you generally aren't actually saying something is of the Devil or calling on God when coming across a mildly surprising situation. They are merely codified expressions.
Fek! certainly is useful, just as the slightly longer kio la fek!? used (a lot) by Evildea, the famous YouTuber Kion la Fek' Mi Ĵus Ludis?
Dissaltu! is translated as "get lost". I have seen iru kacen (slavic inspiration) instead of iru al inferno on Telegramo. Simply kacen or bugren probably work too...
Otherwise I like fulmotondro! because it is neither obscene nor religious. (Unless you would attribute it to Thor or some other mythic thunder god.)
Less simple and longer is Aktoj de la Akademio! and it is neither obscene nor religious. I recently used sable! while I was at the beach, but that is hardly widely understood or used. Otherwise Ho ve! certainly does work, but it isn't a swear word - if I had just left it at that, this answer would be pretty boring.
If you look at this wordlist you can certainly find what you're looking for.
It's more like a sound effect, but perhaps "Aĉ!"
It's not offensive, not religious, and conveys an unpleasant opinion of what is being observed.
Terure is a good choice - as is ho ve.
I wonder, though, what you mean by "non religious." Even "golly", "jeeze", and "gosh darn it" are religious in origin.
Most blasphemes are based on religion, sex, or poop. In English, I prefer to swear on Mother Hubbard and Mother of Pearl Inlay - which are ultimately religious references. I also swear on "sons of motherless goats" which is probably a sexual reference, ultimately. How far removed to these references have to be not to count as religious in your book.
For my part, I don't consider damne and diable to be religious, even though it is clearly more religious than Mother Hubbard. (Oh, and let's not forget "hell's bells" - which I say all the time without thinking of hell.)
You could go with a sarcastic vere belete!
There is probably no end to the made up expressions you could use. I often express surprise with sankta bolak! a reference to an early Idist. Zamne from elsewhere in this thread is very clever and worth using.
"Damne!" is also quite common. Not sure if it has a religious reference.
I can't confirm this is used at all. But terure should carry the same meaning without being religious. You may sound "foreign" when using it, because it isn't commonly used in my experience, although it should be perfectly understandable.
The most popular is Fek (Poop). I also think a good one would be Damne (Damn). Or, a very light one would be terure (terrible).