Templates
Since you said "server-side" GUI, maybe you are thinking more along the lines of a template engine for creating HTML? This isn't really restful, but can be MVC like if you are forwarding from your Controller classes.
But PLEASE keep in mind that it would be a very BAD idea to try and generate XML or JSON data for consumption by RESTful clients (ajax) with either of these. Just say no to JSON creation from a template engine!
JSP is pretty much the standard for Java EE at the moment, or is most widely used. All you need is a compliant server, such as Tomcat/Jetty.
http://www.oracle.com/technetwork/java/javaee/jsp/index.html
Thymeleaf It looks a lot cleaner than JSP, and has the added benefit of not allowing all the pitfalls that can happen with a JSP (and there are a lot).
http://www.thymeleaf.org/
JavaScript:
When you say you are creating a RESTful backend, most people would make the conclusion (and rightfully so) that you are trying to develop a JavaScript MVVM client/GUI.
Kendo UI has a complete set of GUI widgets and MVVM framework that is specifically designed to work with a RESTful backend. But it's a paid for API.
http://www.telerik.com/kendo-ui
YUI3 (Yahoo User Interface) has a variety of widgets as well, and I've seen a lot of jobs requesting this. It has the advantage of being free/OSS as well. Older version are deprecated.
https://github.com/yui/yui3/wiki
knockout.js has been my favorite for MVVM engines that I've actually had experience working with. And it's simple to learn, in my opinion.
http://knockoutjs.com/
Angular.js Also free/oss, and works well with other GUI APIs.
https://angularjs.org/