CSS only drop-down menu not using UL/LI

i'm a newbie / self-taught html guy. I need to edit my (static) website's menu to make it mobile friendly for google within the next 4 days. I'm breaking my head trying to figure out how to do it, but no luck so far, so I'm getting desperate. The way I'm making my entire website mobile friendly is by setting the viewport and changing the CSS if screen width is less than Xpx.

I need to use ONLY CSS to transform the following menu, to a menu button. ie in mobile view, have the entire menu hidden within a button at the top of the screen and make the menu appear/dropdown by clicking on the button.

This is a sample menu I have on my website: www.oikotrust.gr/en And here's the mobile friendly one I managed so far (uploaded it here just for reference): http://ift.tt/1D8gKoJ

<div class="menu">
<div class="menu_btn here">ΑΡΧΙΚΗ ΣΕΛΙΔΑ</div>
<div class="menu_btn"><a href="info.html">ΕΝΟΙΚΙΟ</a></div>
<div class="menu_btn"><a href="photos.html">ΦΩΤΟΓΡΑΦΙΕΣ</a></div>
<div class="menu_btn"><a href="contact.php">ΕΠΙΚΟΙΝΩΝΙΑ</a></div>
</div>

related CSS in original page

.menu{margin-top:50px; float:left; margin-left:none;}
.menu_btn{height:35px; font-weight:bold; padding-top:10px; width:100%; padding-left:20px; font-size:18px;}
.here{background-image:none; background-repeat:no-repeat; color:#E5541C;}

All and any help much appreciated!