People mostly leave you to your own devices at the park. You can be there juggling, dancing, hula-hooping, or singing. People won't interfere with you. Everyone is busy doing their own thing.
If you're nervous about going by yourself, take a friend. They can give you some moral support while you practice.
It's also a great place because:
You get fresh air
You can practice your vocal projection
You can spend a few moments in quiet contemplation
Once again, if you're nervous, go at a time when there's not a lot of people around. There's often a lot of early morning joggers, and people directly after work. Sometimes you'll find that the middle of the day is a good time.
If there's a problem going to the park, you can probably find some woodland where you can practice. There probably aren't other people around for miles - and you can keep the local flora and fauna entertained.
Go to the park and practice. It's great fun! And you can enjoy nature at the same time!