By 'cool' I assume you mean 'good looking' not a lower temperature? 8)
Everyone will have a different idea and the suggestions could be endless.
What sort of fish do you like? Lots of small ones or slightly larger fish? You are limited a bit by tank size of course.
Once you have chosen the fish you like, where do they originate? Look up what plants would be found in their natural habitat and set up a biotope for their region.
Remember a couple of guppies, unless they are both male, will turn in to LOTS of guppies fairly quickly.
Most "community fish" go with guppies but watch out for those with a rep for nibbling as they will like the guppy tails.
I like cherry barbs and rummy noses. Both would be fine with guppies. A larger group of one species will look much better than a few of this and a few of that species.
I once saw a tank of discus (6ft I think - the tank, not the discus) with the whole back half planted, end to end, in straight val. It looked stunning! The straight lines of the val were a lovely counterpoint to the round shape of the discus.