kyou ha kontakuto de ha naku, megane de dekakemasu.

今日は sets the topic: as for today or today.

In Japanese, は is often used to introduce the topic of the sentence rather than mark the grammatical subject. So here, 今日は means something like:

• As for today,
• Today,

It frames the rest of the sentence as something true for today in particular.

コンタクト is a very common shortened form of コンタクトレンズ in everyday Japanese.

So:

• コンタクト = contact lenses / contacts
• コンタクトレンズ = contact lenses (full form)

This is similar to how English speakers often say contacts instead of contact lenses.

ではなく means not ... but ... or rather than ...

In this sentence:

• コンタクトではなく、眼鏡で
  = not with contacts, but with glasses

It contrasts two options and says the second one is the actual choice.

A useful way to think of it is:

• Aではなく、B = not A, but B

Here, で marks the means, tool, or condition by which the action is done.

So:

• コンタクトで出かける = go out wearing contacts
• 眼鏡で出かける = go out wearing glasses

Even though English says with glasses or in glasses, Japanese often uses で for this kind of practical means/state.

You can think of it as:

• using contacts
• using glasses
• more naturally in English: wearing contacts / wearing glasses

This is because では is made of:

• で
  • は

The で belongs to コンタクトで, and then は adds contrast/emphasis. Together, ではなく is a common pattern.

So:

• コンタクトで = with contacts
• コンタクトではなく = not with contacts

The は helps create the contrast with 眼鏡で later in the sentence.

Yes. A more casual version would be:

• 今日はコンタクトじゃなくて、眼鏡で出かけます。

This means the same thing.

Roughly speaking:

• ではなく = more formal or written
• じゃなくて = more conversational

So the original sentence sounds a bit more neutral/polite, while じゃなくて is very common in speech.

Japanese often leaves that idea implicit when it is obvious from context.

Instead of saying:

• 眼鏡をかけて出かけます = I go out wearing glasses

the sentence says:

• 眼鏡で出かけます

This is more compact, but it still naturally means go out in glasses / wearing glasses.

Likewise, with contacts, the act of wearing them is understood from context.

Because they come from different sources:

• 眼鏡 is a native/Sino-Japanese word usually written in kanji.
• コンタクト is a loanword from English, so it is written in katakana.

This is very common in Japanese. Loanwords are usually written in katakana even when there is a Japanese equivalent.

The comma helps separate the contrasted parts:

• コンタクトではなく、眼鏡で

It makes the sentence easier to read and shows a pause, similar to English:

• not contacts, but glasses

In Japanese, commas are often used more flexibly than in English, so this comma is helpful but not always strictly required.

出かけます means to go out, to head out, or to leave home to go somewhere.

It focuses on the act of going out from where you are, especially from home or your current place.

By contrast:

• 行きます = go
• 出かけます = go out / head out

So this sentence is specifically about what the speaker will wear when going out.

Often, yes—or at least it can suggest that.

Because it begins with 今日は, it can imply:

• Today, I’m going out in glasses
• maybe unlike other days, when I might wear contacts

That contrast is not always strong, but 今日は can make the choice sound a little specific to today.

Yes. Here are some common alternatives:

• 今日はコンタクトではなく、眼鏡をかけて出かけます。
  More explicit: Today, I’m going out wearing glasses instead of contacts.

• 今日はコンタクトじゃなくて、眼鏡で出かけます。
  More casual.

• 今日は眼鏡で出かけます。コンタクトはしません。
  Split into two simpler sentences.

The original sentence is natural and concise, especially in standard written or polite spoken Japanese.