As with contemporary window treatments, keep it simple.
Curtains with an attached floppy valance work well (the valance can be in a contrasting fabric), as do pencil and pinch pleats.
Fabrics to consider are fresh floral printed cottons and chintzes, checks and stripes - ginghams for example - don't be afraid to mix these.
Poles can be oak or pine with simple finials.
Keep away from fussy trimmings - consider simple braids or button braids.
Tiebacks in the same fabric as the curtains.