Gday Jack
To obtain this outcome on your first diorama, I'd say you have a pretty creative mind. That said, for your next have a bash at some research and photographs before a build. You'd be amazed at how a good read on certain events/battles gets the creative jooces flowing
It appears your base is on styro foam? Easily breakable mate. For a simple yet effective base, why not try a cheap photo frame you could pick up for a quid or two? Ready made borders, and in varying sizes, and then you can add 'retaining' walls of balsa wood, and then add your foam for height within it. Coat with some plaster of paris, and you have quite an inexpensive base

As for your scene, an easy way to make more action is to make the shell craters 'fresh' as Charles mentioned, by adding scattered dirt clumps, maybe breaking a leg off the table and have it half overturned or similar... Suddenly the action is created because of either the panic or cool assessment of the situation by those involved amidst the shelling?
Keep us posted
Brad