You don't need an event planner, unless of course your doing some bullshit wine tasting for a bunch of BMW riders and their business partners. Haha.
Most importantly, cover your own ass. Insurance, security, etc. You don't need cops and armed guards but a few big dudes (who you trust and know) to keep an eye out for bullshit.
Advertise NON STOP!!!!!! Facebook, a website, but for the big impact get out and shake hands. Hang flyers everywhere. Hit up the local bars and music venues. Tell anyone who will listen.
Also, if you plan on having a band, get something the majority of folks will dig. Not everybody loves metal are street punk stuff. A good, well known cover band that doesn't play only radio bullshit is always a decent call. Not the most inspiring, but it keeps most of the people happy all the time.
If you can get sponsors, milk them dry. They have them money and budgets for events WAY bigger than yours. Make them very aware that this isn't some flash in the pan deal. A little persistence and a lot of phone time can get you everywhere. Play your cards right and the sponsors should cover a lot of the work. If nothing else, advertising, entertainment, and possibly food can be covered almost entirely by the sponsors. Now, if it's some small local business that's signed on this obviously doesn't apply. But, the big companies... Get what ya can while ya can.
As DrJ said, DON'T call it a "cafe" or "rocker" or any other cliche term show. The best shows I've ever been to for cars or bikes are the ones that don't bury themselves in one niche. Do you think DoTheTon would be what it is today if Tim set a CAFE RACERS ONLY sign in the virtual front yard? Hell no. Let them all come and do their thing. Also, don't be one of those shows that segregates different bike styles unless it's needed for judging or something.
Keep shit chill and laid back. Get as many women to come as possible. Don't serve booze and no food. Plenty of bathrooms. Close enough to the city that everyone will come, far enough from hustle that everyone can have fun.
Good luck man, and keep us posted.