Most hot tubs come with a cover included, which can be very useful for insulating and maintaining the temperature when the tub isn't in use, as well of course for covering the open water from debris and making it safer for children. They're also make life much easier when it comes to hot tub winter care.
Along with a cover you might also want to think about a hot tub shelter for over the top of the hot tub whilst you're in it. This can be especially useful on sunny days when you want some protection from the rays, or some shelter from the drizzle!  As an alternative to a permanent canopy, check out the best gazebos for some budget buys.