There is no need to ever run out of things to test on your website if you are in a market that has at least a fair amount of competition. The competition will give you plenty of things to do and not do.
When you find something on a competitors site that you don’t have, make a note of it and test it. You never know when you will come across something that will make a huge difference to your bottom line.
Here is a list of things to test, some will be relevant to your site some will not:
Calls to action
Email capture mechanisms
Free report offerings
Telephone number display
Descriptions of product
Buttons (size, copy, colour)