You can type commands into the OpenSim window. It can be a bit daunting if you are not used to 'command line' applications, and although there is a possibility that you might completely wipe everything that you've made, the chances of doing it by accident are very slim (hint: avoid any commands that include the word 'delete').
To list the available commands type: help
To create a flat terrain (taken from Roger Stack) type: terrain fill 21
If you're tired of being Test User type: create user firstname lastname
When you're finished and want to shut down type: quit
Note that permissions are ignored by default, so even if you login as a different avatar you will still be able to modify everything that's there. You won't have access to the other avatar's inventory though - unless you feel brave and use the IAR system to transfer it.