It depends on the water you will be running on. If it is clear, you can set up some neat underwater obtacles using hula hoops. Fishing line and heavy weights will keep them in place. You can sometimes get tire weights and other heavy objects for free from your local automotive repair shop (brake rotors are good weights)
For buoys, you can use 1 liter soda bottles tied by the cap end, and filled with enough water to get the height you wish. You can paint them red or black if you want to get fancy.
From there, anything goes. You can have piers to practice your docking skills, islands with structures on them made from foam and detailed like Model railroad layouts, ships anchored out in the water. A hula hoop with large chunks of foam can be an impromptu ice flow to practice precision surfacing. You can set up a production line with your other club members to make realistic buoys - making a pattern and casting multiple copies out of resin would be a natural here.
I always thought that a really neat club would require each memeber to be responsible for some scenic element that could be brought to a fun run or regatta, as well as a couple of buoys - a pier, island, sinking ship, or something else. Have a contest to see who brings the most inovative decoration each time. The net result will be a very enjoyable running enviorment.