How to grab the values of field collection items in a template file?
It turned out that I was making things too complicated: the theming can be achieved with CSS by using the default classes of the field collection. Hopefully, my code underneath can be helpful for others too.

.item-list li {
  list-style: none;
}

.field-name-field-plaats:before {
  content: "@ ";
}

.field-name-field-plaats {
  display: inline;
}

.field-items {
  display: inline-block;
}

.field-name-field-aantal:before {
  content: " (";
}

.field-name-field-aantal:after {
  content: "x)";
}

.field-name-field-aantal {
  display: inline;
}