<nav>
  <ul class="menu">
    <li class="menu-item"><a href="#">Home</a></li>
    <li class="menu-item"><a href="#">Holidays</a>
      <ul class="submenu">
        <li class="submenu-item">
          <ul class="submenu-top">
            <li class="submenu-top-item">Holiday types (links with thumbnail images)</li>
            <li class="submenu-top-item">Holiday packages (links with descriptions)</li>
            <li class="submenu-top-item">Our services (links with icons)</li>
            <li class="submenu-top-item">Last minute offers (links)</li>
          </ul>
          <!-- End .submenu-top -->
        </li>
        <!-- End .submenu-item-->
        <li class="submenu-item">
          <ul class="submenu-bottom">
            <li class="submenu-bottom-item">Trending destination 1</li>
            <li class="submenu-bottom-item">Trending destination 2</li>
            <li class="submenu-bottom-item">Trending destination 3</li>
            <li class="submenu-bottom-item">Trending destination 4</li>
          </ul>
        </li>
        <!-- End .submenu-item -->

      </ul>
    </li>
    <li class="menu-item"><a href="#">About</a></li>
    <li class="menu-item"><a href="#">Blog</a></li>
    <li class="menu-item"><a href="#">Contact</a></li>
  </ul>
</nav>

/* General styling */
* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
}
body {
  background: #f1f1f1;
}
.menu li {
    padding: 10px;
}
ul {
    list-style-type: none;
}
a {
    color: white;
}
img {
    max-width: 100%;
}
/* Flexbox rules */
.menu {
    display: flex;
    align-items: center;
    text-align: center;
    width: 90vw;
    margin: 20px 5vw;
    height: 60px;
    position: relative;
}
.menu-item {
    flex: 1;
}
.submenu {
    width: 90vw;
    position: absolute;
    top: 60px;
    left: 0;
    text-align: left;
    display: flex;
    flex-direction: column;
}
.submenu-item {
    padding: 15px;
}
.submenu-top {
    display: flex;
    justify-content: space-around;
}
.submenu-bottom {
    display: flex;
}
.submenu-bottom-item {
    flex: 1;
}
/* Colors */
.menu {
    background-color: darkslateblue;
}
.submenu {
   background-color: #bbb;
}
.submenu-top,
.submenu-bottom {
    background-color: turquoise;
}
.menu-item {
    border: 3px solid turquoise;
}
.submenu-item {
    border: 3px solid mediumvioletred;
}
.submenu-top-item,
.submenu-bottom-item {
    border: 3px solid black;
}