Definitely need to have roses and horses! Pick up some inexpensive horses and surround with roses for a perfect centerpiece! Source and tutorial
Purchase Dollar Store horses, spray paint and add a paper rose! Source and tutorial
If you want to go a little more playful... these cookie "horses" are genius! You could stash this one for a Barn party, too! Source and tutorial

You can't have a derby party without fancy hats! These marshmallow and cookie hats would be a great way to end the evening! Source and tutorial