How many variables (input) do you need? How much time do you intend to use with this program? Is this for use during customer consulting? What level of visual interface do you want? What level of visual output? How complex are the designs?
You have three big options:
1) Use a scripting language (like PHP) with a web browser (like Apache) and a database (flatfile or otherwise). You can use simple HTML to make your interface. This can of course be done locally - no network connections. But you could access it from the Internet if you wanted. Clients could even do so, if you were so inclined.
2) Use a scripting language entirely. You can use GUI libraries, but this adds a level of complexity. The storage of data will require a flatfile database (at least), so you don't really gain a real advantage over the other the web imitation in #1. Of course, a GUI library might also provide you with some interactive elements similar to a more mature program.
3) Use a more heavily minded language like Java. You can do whatever you want, but it will take more work than you are probably willing to put in.
Number #1 sounds good for you. You can use Javascript to make some interesting designs if you want - plus HTML is easy so you have some familiar ground. A PHP backend (or whatever you want, there are many options here) is standard fare - even with a application database (like MySQL). There are several books which cover this topic pretty much completely. Most importantly, changes can be made quickly and you can avoid any semblance of software development while still making something you can use.
