Create a control/view system for your project, for example: an avatar moving around, a script that allows you to move units for an RTS, a window and text messaging interface for an UI centered game.
It can use placeholder assets. It doesn’t have to include content like a level or text but it should include a sample of the environmental elements you need to experience the core interaction, for example: if it’s a platformer include some platforms, if it’s a card game have decks and drop areas to move the cards around.
Make a kind of playground that gives a sense of the “game feel”.
The whole team should be involved in designing and testing the project, even if the scripting is done by one person.
You can (should?) produce more than one control prototypes.
If the implementation is straightforward or you don’t have much to do, you can start fleshing out some other aspects of the game.
I’ll help you define team-specific goals.