Well, as mentions making the final volume less, with the same ingredients will increase the flavour(and ABV).
Alternately, for a more malty flavour you can add some light DME, maybe another 500g,
You could also steep some specialty grains, ie 250g of Caramalt. This is a great way to add that 'fresh' feel to your beer.
Adding hops will increase the flavour. This is done by selecting a hop you want, for example 20g Cascade steep it in hot water for say 10 minutes, then add this to your fermenter before pitching your yeast. this may increase bitterness if you use hot water above 70 degrees Celsius. If you are steeping gains then you can add hops to that process, after you've finished steeping the grains.
You can then also dry hop - ie by adding 20g of hops to the fermenter after the first 4 -5 days and leaving it for a week or so before bottling.
All these amounts of grain and hops are up to you to choose, but the above mentioned amounts are a good place to start.
If you are after more bitterness you can boil hops for up to 30 - 60 minutes in either the wort created from steeping the grain or one made from some of the can goop or other dry ingredients.
Ian's spread sheet is a great tool in help you create a good extract beers
http://aussiehomebrewer.com/topic/29655-kit-and-extract-beer-spreadsheet/
I hope this helps Axl100