There is no stopping creative content!
You can take up the challenge and try any ideas you wish to have a try at.
Look at how the CSS works.
Look at the code and make changes to border colours on tables which can only (unfortunately) be done in the HTML code, using the HTML commands, winch can look something like this: <table width="309" border="1" align="center" cellpadding="0" cellspacing="0" bordercolor="#cc3300"> (you add the bordercolor="#cc3300" to the code tag. The '#cc3300" is the hexadecimal colour code. Go to http://www.w3schools.com/tags/ref_colorpicker.asp , to see the complete set of colour codes.
Create a rollover image or several. See Images.
Place background images in to fill the screen.
Try the Mouseover21 gallery and link it to your site. Mouseover21 is previewed here.
And with your skills and imagination, there could be a lot more! Do your research by looking at as many websites ans you can and learn skills.
Javascripting opens up a whole new area of learning, whether it be adding scripts or menu bars. There are many useful (and useless) free Javascripts out there, just Google Free Javascripts and do your research.
Include a video or Youtube movie or two - but ensure the content is relevant to your themed topic.