Two things that have greatly increased my balance is a balance board workout routine and Brazilian Jiu Jitsu
The balance board I picked because I have a gimped up ankle. I had broken it a couple times and required 5 surgeries on it in total. That was years ago and it's healed, for the most part. The problem I was having was that I was injuring it very easily. Usually it was just minor things like twisting during pickup basketball games or sprains when caught in toe holds or ankle cranks. I finally got sick of it happening so I asked both a doctor and physical therapist what they would recommend. They both said that if I build up the strength in it, the injuries should subside, and both suggested getting a balance board. If you haven't seen one of these things, it's just a round board with a half sphere centered on the bottom of the board. You stand on it and then go through a series of movements such as touching the front of the board to the floor and rocking back so the back is touching the floor without letting the sides touch. Then you just try to keep all of the edges off of the floor for as long as possible. Things like that.
When you first start using it, it's hard to do. It's impossible to keep yourself balanced for more than 30 seconds or so and the fronts of your shins, calfs and ankles fatigue really quickly, but after a couple weeks, you see noticeable improvements.
Brazilian Jiu Jitsu is just fun as hell, but it also requires a lot of body control and a good grasp of leverage. You have to be able to contort your body in different directions while also fighting against forces being exerted on you from different angles in order to maintain a steady base so you can attack, defend or move to more advantageous positions. I don't know how much that kind of balance would translate to disc golf, but I imagine some of it does. It also works your core like no other workout I've ever done before and having a stronger core is beneficial for any kind of physical activity. The only downside is that it can put some strain on your joints. There have been times when my elbows were too sore to play disc golf the following weekend, but it's only happened a couple times over the course of several years.