kono ryokoukeikaku ha genzituteki da to omoimasu.

In Japanese, subjects like “I” (私) are often left out when they’re obvious from context.

この旅行計画は現実的だと思います。
literally: As for this trip plan, (it) is realistic, I think.

Because you are the one speaking, the listener automatically understands that you are the one who thinks this. So adding 私は is usually unnecessary:

• (私は) この旅行計画は現実的だと思います。
  → Grammatically fine, but 私は is often omitted in natural conversation unless you need to contrast yourself with others or avoid ambiguity.

は is the topic marker. It tells us what the sentence is “about”.

• この旅行計画は 現実的だと思います。
  → “As for this travel plan, (I) think it is realistic.”

So the structure is:

• [Topic] は [Comment]
  
  • Topic: この旅行計画 (“this travel plan”)
  • Comment: 現実的だと思います (“I think (it) is realistic”)

You could roughly feel it as “As for this trip plan…” or “Regarding this trip plan…”, though English doesn’t always say that explicitly.

You need だ because 現実的 is a na-adjective, and before と in patterns like 〜と思う, na-adjectives behave like nouns and take だ:

• Noun / Na-adjective + だ
  • と思う
    
    • 現実的 (na-adjective) → 現実的だと思う
    • 便利 (na-adjective) → 便利だと思う
    • 静か (na-adjective) → 静かだと思う

So:

• 現実的だと思います。 ✅ natural
• 現実的と思います。 ❌ sounds ungrammatical/unnatural in standard Japanese

For i-adjectives and verbs you don’t add だ:

• 高いと思います。 (i-adjective)
• 行くと思います。 (verb)

です almost never goes directly before と思う. The normal pattern is:

• [Plain form] + と思います

So for different word types:

• Noun / Na-adjective:
  
  • 現実的だと思います。
  • 先生だと思います。
• I-adjective:
  
  • 高いと思います。
• Verb:
  
  • 行くと思います。

です is the polite copula that appears at the end of a sentence:

• この旅行計画は現実的です。
• この旅行計画は現実的だと思います。

So:

• 現実的ですと思います。 ❌ unnatural
• 現実的だと思います。 ✅ correct

In 〜と思う, the と is a quotative particle. It marks what you are “quoting” as your thought:

• [I think] “this plan is realistic.”
  → The quoted part is: この旅行計画は現実的だ
  → In Japanese: この旅行計画は現実的だ と 思います。

So the pattern is:

• 「Sentence in plain form」 + と + 思う

The と here doesn’t mean “and”. It’s more like putting quotation marks:

• A は B だ と 思います。
  = I think “A is B.”

• 現実 (genjitsu) = “reality” (a noun)
• 現実的 (genjitsuteki) = “realistic” (a na-adjective meaning “practical / feasible / realistic”)

In this sentence you want to say the plan is realistic, not that it is reality.

• この旅行計画は現実だ。
  → “This travel plan is reality.” (odd in this context)
• この旅行計画は現実的だ。
  → “This travel plan is realistic.” (what you want)

So 現実的 is the natural choice.

Both are grammatically correct, but there is a nuance:

• この旅行計画
  
  • A compound noun: “this travel plan” as one unit
  • Sounds like a specific, perhaps already defined plan document / proposal
• この旅行の計画
  
  • Literally “the plan of this trip”
  • Feels more like “the planning for this trip” in a more general sense

In many real contexts, both could work:

• この旅行計画は現実的だと思います。
• この旅行の計画は現実的だと思います。

You’ll see compound nouns like 旅行計画, 旅行案, 旅行プラン, etc. very often in written Japanese and formal contexts.

Yes. In conversation, dropping は and using a pause or comma is common:

• この旅行計画、現実的だと思います。

This sounds slightly more casual and “spoken”.
Functionally, この旅行計画 is still the topic, just without the explicit は. It’s similar to starting in English with:

• “This trip plan— I think it’s realistic.”

So:

• With は: more standard, complete sentence feeling
• Without は: more conversational, but very natural

Grammatically yes, but stylistically it’s a bit heavy because you get two は:

• 私は この旅行計画は 現実的だと思います。

This double-topic structure is used, but usually when you really want to emphasize:

• “As for me, as for this trip plan, I think it’s realistic.”

More natural options in most cases:

• 私はこの旅行計画が現実的だと思います。
  (私 = topic, 旅行計画 = subject)
• この旅行計画は現実的だと思います。
  (drop 私 because it’s obvious)

So your sentence isn’t wrong, it just sounds more marked or contrastive than needed in a neutral context.

In standard polite Japanese, no: you should keep だ:

• この旅行計画は現実的だと思います。 ✅ natural
• この旅行計画は現実的と思います。 ❌ unnatural

For na-adjectives and nouns before と思う, だ is normally required.

You may sometimes hear 現実的と思う in very casual or dialectal speech, but as a learner you should treat だ as necessary here.

The difference is just politeness level:

• 現実的だと思います。
  
  • Polite form (〜ます)
  • Use with people you don’t know well, in formal situations, etc.
• 現実的だと思う。
  
  • Plain / casual form
  • Use with friends, family, or in informal writing

Meaning is the same: “(I) think it’s realistic.”