にを particle?
 

if i want to say please show me what's in your bag, do i use

1) かばんのなかをみせてください or
2) かばんのなかにをみせてください

The verb "to show" is みせる　（見せる）

かばんの中を見せてください。

What MMM said.

Is it just me, or do 中を見る sound like "to look inside" and 中に見る like "to look in the general direction of the inside"?

thanks for pointing out the error mmm, i've edited the question

中に見る makes no sense. We just don't say it.

中に～～が見える, however, is correct. A fortune teller might say 「クリスタルの中にカウボーイが見えます。恐らくあな たはアメリカ人と結婚するでしょう。」;)

is it used like this:

かばんの中にえんぴつが見せてください

or

かばんの中にのえんぴつが見せてください

in this sense, does the entire かばんの中に acts as an adjective describing the pencil?

Either of your sentences is correct.

The が/を mistake is huge. Some particle mistakes in Japanese look as grave as word order mistakes in English.　It was only a day or two ago when saw you use を correctly. I wonder if you aren't learning too rapidly without proper reviewing.

かばんの中のえんぴつを見せてください。

You simply cannot say かばんの中に unless you are doing things like "putting something" or "tossing something" into the bag.

Use の wisely and you will be the master. I call の the magic particle.

Read the following phrases carefully. おり= cage. 空気= air.
かばんの中のえんぴつ
おりの中のライオン
家の中の空気(くうき)

oic, also regarding the の particle, is it true that sometimes its optional. if so how do i decide whether its required or optional?