You can use :symbol
as the first argument.
<%= simple_form_for :user, url: users_path do |f| %>
<%= f.input :name, as: :string %>
...
<% end %>
It will output something like this:
<form novalidate="novalidate" class="simple_form user" action="https://stackoverflow.com/users" accept-charset="UTF-8" method="post">
...
<div class="input string required user_name">
<label class="string required" for="user_name">
<abbr title="required">*</abbr> Name
</label>
<input class="string required" type="text" name="user[name]" id="user_name" />
</div>
...
</form>