I always have lots of stuff to charge (camera, strobe, UW lights, etc), and would appreciate a 'charging station'. I envision a table or cabinet with multiple spaces, with multiple power outlets all over. At home, I choose my power strips carefully, so they can accommodate plugs and bricks of different sizes and orientations. Maybe even include an array of high-power USB sockets too.
Now I'm daydreaming: If the 'charging station' had some robust doors and a lock, it could be used for secure storage of tablets and computers while away. Alternatively, if you provide a safe, it would be awesome to have power/usb inside!!
Many here have already talked about drying racks, or lockers, or gear storage. Make sure there is lots of airflow or even a fan. Let me summarize: someplace secure where I can wash/dry/store my gear, all at once. A dedicated room or shed with a dehumidifier would be really cool. I can imagine a small building with tile, drain, and hangars/racks inside; rinse tanks, a hose, and a shower outside. A big locking door if it has AC or dehumidifier, or a big locking gate if it is 'open air' storage.
If shore diving via car/truck, maybe layout a convenient parking space where gear can be loaded/unloaded quickly and easily. It would be cool to open a large door and have the truck bed within a few feet.
How about 'dive towels' separate from bath towels. I often wish for a nice soft fluffy bath towel after showering, but also want something (cheaper) that I can take to the pool or diving.
If you do something radical like including a car/truck in the rental, having a locking storage in the vehicle would be appreciated. See multiple threads about vehicles in Bonarie and security for examples of what might also happen in Curacao.
I like the idea of an outdoor shower. If you can't do rinse tanks, maybe a nice convenient hose would be appreciated.
Any maps or guides to nearby dive sites would be a plus, including photos and drawings.
Generic rental feedback:
Although it is getting better, I have had really bad experiences with wifi in rentals. Having a high quality router, and reliable internet is a must. Having a local contact number of someone who can actually come and fix any wifi issues is BIG for me.
I love having a grill, but HATE how dirty they look. It often is a huge turn-off, and I avoid it. Other peoples grime is gross. Be sure your cleaning service is instructed to diligently and excessively clean it!!
If you intend to market to families, have some video games setup.
TV: More channels is better!! Just because you like and buy the 'European Water Polo' channel, doesn't mean that your guests will feel the same. Quantity+Variety!
Pool: if you have one, include some toys for kids.