Learn Hardcore Croatian: Za sljedeće putovanje uzet ću samo malu torbu. - For the next trip I will take only a small bag.

The preposition za always takes the accusative case when it means for (purpose, intended use).
Putovanje is a neuter noun; its nominative and accusative singular have the same form: putovanje.
The adjective sljedeće agrees with putovanje in gender (neuter), number (singular), and case (accusative), so it also appears as sljedeće.

Why is the preposition za used here instead of na?

Za in this sentence means for in the sense of “for the purpose of”:

Za sljedeće putovanje = for the next trip (in preparation for it, with it in mind).
If you said na sljedeće putovanje, it would usually mean onto/for going on the next trip, focusing more on going on that trip than on preparation.

In the context of packing or choosing luggage, za is the natural choice because you’re deciding what you will take for that trip.

What tense is uzet ću and how is that tense formed in Croatian?

Uzet ću is Future I (the normal future tense).

It is formed with:

a present tense form of htjeti as a clitic (ću, ćeš, će, ćemo, ćete, će)
- an infinitive of the main verb.

Patterns:

Ja ću uzeti torbu. = I will take the bag.
Uzet ću torbu. = I’ll take the bag. (shortened infinitive, same meaning)

Why is it uzet ću and not uzeti ću?

In standard Croatian, when the auxiliary ću comes after the infinitive, the infinitive normally drops the final -i:

full infinitive: uzeti
shortened form before ću: uzet ću, not uzeti ću.

So:

uzet ću = correct and standard
uzeti ću = considered non‑standard / wrong in Croatian orthography.

When ću comes before the infinitive, then you keep the full infinitive:

Ja ću uzeti samo malu torbu.

Can I also say Za sljedeće putovanje ću uzeti samo malu torbu? Is it different in meaning?

Yes, Za sljedeće putovanje ću uzeti samo malu torbu is grammatically correct and the meaning is the same.

The difference is only in word order and rhythm:

Za sljedeće putovanje uzet ću samo malu torbu. – uses the shortened infinitive (uzet) with ću right after it.
Za sljedeće putovanje ću uzeti samo malu torbu. – keeps the full infinitive (uzeti) and places ću in the typical “second position” after the initial phrase.

Both are natural in Croatian; you will hear and see both patterns.

What does samo modify here, and what would change if I moved it?

In uzet ću samo malu torbu, samo clearly modifies malu torbu:

It means: I will take *only a small bag (and nothing else)*.

If you move samo, you change the focus:

Samo ću malu torbu uzeti. – The only thing I’ll do (in this context) is take a small bag. (emphasis on the action being the only one)
Uzet ću malu torbu samo za sljedeće putovanje. – I’ll take a small bag *only for the next trip.* (not for others)

So placement of samo is important for what is being limited.

Why is it malu torbu and not mala torba?

Mala torba is the nominative form (the one you’d find in a dictionary: a small bag).

In the sentence, malu torbu is the direct object of uzet ću, so it must be in the accusative case:

feminine singular nominative: mala torba
feminine singular accusative: malu torbu

The adjective (mala → malu) and the noun (torba → torbu) both change to reflect the accusative.

Why is the adjective sljedeće and not sljedeći or sljedeća?

The form of the adjective must match the gender, number, and case of the noun it describes.

Putovanje is neuter, singular, accusative.
Neuter singular accusative of sljedeći is sljedeće.

So:

sljedeći – masculine (e.g. sljedeći vlak)
sljedeća – feminine (e.g. sljedeća stanica)
sljedeće – neuter (e.g. sljedeće putovanje)

That’s why the correct form here is sljedeće putovanje.

What is the base/dictionary form of uzet, and why is the final -i missing?

The dictionary (infinitive) form is uzeti = to take.

In Croatian Future I, when the auxiliary (ću, ćeš, …) is placed after the infinitive, the infinitive usually drops the final -i:

uzeti
- ću → uzet ću
napisati
- ću → napisat ću

So uzet is just a shortened infinitive form used before the clitic ću, not a different verb.

Could I say ponijet ću samo malu torbu instead of uzet ću samo malu torbu? Is there a difference?

Yes, you can say Ponijet ću samo malu torbu, and it is very natural.

Nuance:

uzeti – to take in a general sense (to choose, to take from somewhere). In this sentence, it can suggest I will choose to take only a small bag (for the trip).
ponijeti – to take (along), to bring with you. This emphasizes that you are carrying the bag with you on the trip.

Both are fine in context; ponijeti may sound slightly more focused on the act of carrying the bag on the journey.

How do you pronounce sljedeće? The slj cluster looks tricky.

Sljedeće is pronounced roughly like:

sljé-de-che (with lj as a single sound, similar to the ll in Spanish llama in many accents).

Details:

s: as in see
lj: a palatal l (tongue high and forward; not slye, more like slye with a soft l)
dje: like dje in djeca (sounds like dje with a soft d)
će: će is che (like ch in chew, but softer), with a short e.

So the stress is on the first syllable: SLJÈ‑de‑će.

Is there any difference between Za sljedeće putovanje uzet ću samo malu torbu and Uzet ću samo malu torbu za sljedeće putovanje?

Both are correct and mean essentially the same thing.

Za sljedeće putovanje uzet ću samo malu torbu. – starts with the purpose (for the next trip), then states what you’ll do.
Uzet ću samo malu torbu za sljedeće putovanje. – starts with your decision (I will take only a small bag), and adds za sljedeće putovanje at the end as extra information.

The difference is just in information order and emphasis, not in grammar or basic meaning.