Your question is relative to each individual and their goals. With regard to client programming, I agree with Harris and Daniel. You must first look at your clients individually and their goals. Then take your education and knowledge, and come up with some “tweaks” to their programming that may make it more fun/challenging/interesting for them.
Sometimes, we can find change in just the little things. Use different pieces of equipment. Or, make little tweaks for bigger results. For example– in strength training, you can play with reps, tempo, eccentric focus, hand grip, supersets, tri-sets, etc…
Or,take them to another location. A different room, environment, outdoors, etc.. Get them out of the normal location. It does wonders for their spirits and breaks up monotony.
Also, I would look at the personality of each client. Do they like structure or are they adventurous? Do they like music? You can think of a thousand ways to shake it up for them. That being said, once you have an idea on each client, then search the various resources out there for new content. 🙂
You will find some great content here on IDEA both under articles and on the IDEAfit TV.