Is it ok to leave items in the trunk of a car?

I drive a 1999 Honda Civic and i was wondering is it okay to leave items like washer fluid or a case of water or anyone items (other than food) in the trunk. I do not have windows where you can see the trunk but i leave it outside of my house since there's no room in the garage and usually the temperature is around 20 degrees Celsius during the summer. Will it go bad or i should be fine?

Does header makes car sound louder?

I have a Honda civic ex,
I have a straight pipe 2inch or 2 1/2 cat back exhausted with a megna flow muffler, with stock cat. I was wondering if I get headers for my car will it be louder then before or high dp or will get pull over quicker?

98 Honda Civic speedometer?

So my speed gauge is jumping all over the place I change speed sensor and still jumps everywhere when I drive. Does anyone know what's the problem

Car vs suv for under 15k?

I need help deciding wether to get a car or suv. I would like the car/sub to be reliable and parts cheap also easy to customize. Any suggestions?!
Ps. Please don't say Honda Civic or Toyota Corolla.etc…

Added (1). I'm in high school and need a cool car that is easy to customize and insurance is low