I have a web design and want to show some variations on it to some people I'm working with.
What are some simple changes I quickly could make to my current CSS stylesheet to dramatically alter the design in an aesthetically pleasing way and present various alternate presentations of my current design?
The simplest changes are generally to colour and background colour. If your site is a light colour theme (light background, dark text), reverse this. It will make a big impact.
Another simple change is to update all the fonts. For instance if you're mainly using sans-serif fonts (Arial, Verdana), change over to serif (Georgia is an excellent choice for this).
I guess that'd depend a lot on what the current design looks like. However, really simple things like tweaking margins and adding line-height often makes a big difference in pure readability.
Going beyond colors etc, you could use CSS to switch your nav from horizontal to vertical. With good XHTML and plenty of class names and IDs you can change a lot.
Small changes big difference:
Take a look at Zen Garden. It show numerous examples of how the appearance of fixed HTML is changed by supplying different CSS's. While most of the CSS changes are probably major, it certainly gives you some great examples of what can be done.