This place is amazing for those who want to try out gachapons! They have hundreds of different machines and update them regularly. If you are also into rements and figurines, you are also able to buy some here too. All the machines use coins so make sure you bring some cash along (you can exchange notes for coins here).
This place is absolutely amazing and is a must go place!
I recommend going either very early in the morning or in the late afternoon (around 4-5pm) as there will be less people. You will be more likely to be able to take photos with no -one else in the shot. The higher up the mountain you go, the less people also (though it is quite a hike so make sure you wear comfortable shoes!)
I would recommend buying one of the express passes if you plan to go to to USJ, especially if you want to spend time in the Harry Potter area earlier on in the day. You will be able to choose what time you would like to enter the Harry Potter as well as when you want to go on the rides. If you don't have an express pass you'll have to get a ticket before you enter the Harry Potter area and if it gets too busy, tickets will run out and you won't be able to enter the area at all.