Select the list items inside the nav element and set their display to inline-block.

a {
  text-decoration: none;
}

#wrapper {
  max-width: 940px;
  margin: 0 auto;
}

#logo {
  text-align: center;
  margin: 0;
}

h1, h2 {
  color: #fff;
}

nav a {
  color: #fff;
}

nav a:hover {
  color: #32673f;
}

nav ul {
  margin-top: 0px;
  margin-bottom: 0px;
  margin-left: 10px;
  margin-right: 10px;
  padding: 0px;
  list-style: none;
  display: inline;
}

h1 {
  font-family: ‘Changa One’, sans-serif;
  font-size: 1.75em;
  font-weight: normal;
}

img {
  max-width: 100%;
}

#gallery {
  margin: 0;
  padding: 0;
  list-style: none;
}

#gallery li {
  float: left;
  width: 45%;
  margin: 2.5%;
  background-color: #f5f5f5;
  color: #bdc3c7;
}