A party is a great thing, but a party with a theme is even better. It allows everything to go well together and gives the party a cohesive feel. Also, themes can make the party planning a lot easier as well since you already know what kind of things you will need.
A farm themed party is a cute idea. There is a lot that can be done with a theme like this. It allows you to be creative and have some fun while making things easier. You can have the food made in the theme as well. Some animal cookies or cupcakes are really cute and yummy. The decorations are easy. You just use anything in the party store that looks farm related. One cute idea for this theme is to use hay as filler for the gift bags or baskets to give it that fun farm feel, even if the gifts don't have that theme included.
The cake can be easy or more complex, but either way you go it can be a blast. If you do it yourself or have it professionally done, you have two main choices. Firstly, you can go with a simple sheet cake with a barn theme drawn onto it in a fun way with the name of the honored guest. You could also do a tiered cake to give it more depth and have the hay and grass on the bottom, then the animals on the next tier and a barn on the very top. This is better done by a professional as it is much more difficult. The cake is usually the big part of the party theme that everything works around. It is such an important part of the party that you want to ensure a nice themed cake.
This theme can be a real blast. You can have all kinds of deserts made in the shape of farm animals and even play theme games. For example you could play pin the doors on the barn and other theme games to make it more fun. No matter how far you go this theme can be great. You can go all out and make everything farm themed or you could go with a more subtle approach and have a few elements of the theme sporadically around the space. No matter what just remember to have fun planning and at the party.