with (something), (clause)

When you want to tell the context or the reason for a comment, you explain it with the word "with". For example:

With all the rain we've been having, I bet we'll get some great strawberries this year.

In this example, the reason that the strawberries will be great this year is that there's been a lot of rain.

With the time you put into studying, you should do really well on the TOEIC.

In this example, the reason that you think the listener will do well on the test is that he or she has studied a lot.

With all this down time, I don't know what to do with myself!

In this example, the speaker means that she's not sure how to spend all of the down time that she has now.

down time

"Down time" is time when you're not doing work or other necessary activities. Some things that you can do in your "down time" are going out with friends, watching TV, relaxing on the beach, or surfing the Internet. This is a positive-sounding phrase.

It's nice to have some down time, you know, to be able to sit back and relax and enjoy yourself every now and then.

what to do with (oneself)

"I don't know what to do with myself" is a common English idiom which just means that you don't know what you should do. Use this phrase when you have free time and you're not sure how to spend it:

Now that I'm retired, to be honest, I really don't know what to do with myself.

You can also ask someone a question like this:

So, what are you going to do with yourself over summer break?

This phrase sounds colorful and friendly.