How to create a new sandbox page
The beauty of sandboxes is that you can create as many as you need. Create another page off of your Wikipedia user page by editing the URL after your username with the name of a new page you want to create.
For example, to create a new page titled “New sandbox”, first click on
your username in the upper right when you're logged into en.wikipedia.org. Now edit the end
of the URL to look like this:
en.wikipedia.org/wiki/User:Username/New_sandbox
Click Create in the upper right to enter edit mode for a new page.
Once you've added content, click Publish page to save what you've added. (It will still be a sandbox; "Publish" just means save.)
If you forget the URL of this new page, don't worry! You can always check your Contributions history, found at the top right when you're logged in, to find it.