If you are very ambitious, you can think of things that would take close to 30 years to achieve like any kind of scientific discovery, or becoming a senator, or president, or coach to a professional sports team, etc.
I'm not that ambitious, so I have goals that relate more towards the legacy I want to leave behind and the environment I'd want for myself when I'm old. Examples would be things like: I have children who think of me positively, I'm inspiring to others (could specify in what way if you want), I've lived a life of honesty, I've been fair to others, I'm retired, I'm as healthy as can be, I live in the company of loved ones, I have an active social life, I have more then 5 friends I still talk to regularly, I have the means to sustain myself, I'm still active in a hobby, I have no regret, etc.
The 30 year goal basically forces you to define for yourself what it means to have a great and successful life. When I did it, I realized that for me, having real strong meaningful relationships was most important, I couldn't think that 30 years from now anything else would matter to me. So maybe in 10 years I want to become a Principal engineer, but in 30 years I won't care if I did or not, but I'd care if I lost all my friends and sabotaged my social life.
Its toughest to come up with the 30 year goals, but the exercise is worth it I think.