Hat, gloves, boots, body protector, crop anything els you normally ride in but would recommend bring your body protector incase the horse isn't very well behaved!!
to add to all these comments, i would recommend brining your trainer or someone else very experienced who can help you spot any conformation/behaviour/training/health issues that can cause problems in the future
Bring your saddle,helmet,hair net,boots,crop, and a couple pairs of spurs. I always have someone videoing me so I can go back and watch. I also recommend to always do a vet check.
Bring your saddle for the horse and helmet, boots and a couple pairs of spurs/crop for you. I always have someone videoing as well. And as they mentioned below if you like the horse then always do a vet check!
bring your vet and someone to film you and help you. make sure you bring your hat and boots and I would also recommend a body protector. You never know!
Take your usual tack (saddle, boots, helmet…), this will allow you to really feel the horse as he is (a saddle that you are not used to may not reflect accurate sensations). The best is to go with your trainer, and to film the session. You can also have someone film the session and show everything to your trainer after to get some feedback if it is impossible for him to go with you, so take to camera. If the horse interests you after you tried him, as Payton advises you must absolutely do a vet check (and blood tests) afterwards! :)
I would bring your vet along to do a full examination of it and do a drug test to make sure you aren't being sold a dud that will cost you a ton in vet bills in the end