Combining fitness, agility and skill work is a great way to get a lot done in a short period of time.
A simple example would start with a server with a ball and a player 10 yards away behind a cone.
Play would start with a ball to one side of the cone, The player comes forward 1 touches the ball back to the server and then goes back to do the same thing on the other side.
This could be done for a one minute period at full speed.
Next we add the real challenge. Do the exact same thing but this time while on one leg only. This means the player hops forward to receive the ball, passes it back one touch and then hops back around to do the same from the other side. This would be done for one minute and then a one minute rest and then do the same thing with the other leg.
As you can imagine, this same could be done with instep volleys, inside of the foot volleys, headers and many other combinations (chest trap volley back etc). The balance required to do these things will really challenge the players and encourage them to maintain proper technique throughout.
This is an excellent example of how you can take a relatively easy activity and make it much more difficult so that it encompasses many more aspects of the game and fitness with just a little bit of thought.
Have a great day!