activity_form.html.erb

<%= semantic_form_for (@activity ||= Activity.new) do |f| %>
  <%= f.inputs :name => "Quickly record what you did" do %>
    <%= f.input :date, :selected => Date.today, :order => [:month, :day, :year] %>
    <%= f.input :category, :collection => [ "Office", "Personal", "Sales" ], :label => "Type", :input_html => { :class => "change-form-options" } %>
    <%= f.inputs :rel => "Sales" do %>
      <%= f.input :contact_id, :as => :string %>
    <% end %>
    <%= f.input :notes %>
    <%= f.inputs :rel => "Sales Office" do %>
      <%= f.input :outcome, :collection => [ "In the bag", "Will get them with some effort", "50-50 chance no matter what we do", "Probably won't get their business", "Waste of time" ] %>
      <%= f.input :next_steps %>
    <% end %>
  <% end %>
  
  <%= f.submit %>
<% end %>

select_optional_fields_snippet.js

// In some forms the fields change based on change in select
  $('select.change-form-options').change(function(event){
    $select = $(this); //cache the select input
    value = $select.val(); //save the value in the input
    $parent_form = $select.parents("form"); //get/cache the form
    $optional_fieldsets = $parent_form.find("fieldset[rel]");
    
    $optional_fieldsets.hide().filter("[rel~="+value+"]").show(); //~= contains word in attribute somewhere
  }).change();