Tents tables and chairs are a great start, having run many parties for many organizations I have belonged to I can lend this advice.
Be prompt and up front with everything for drop off and pick up.
ALWAYS ALWAYS ALWAYS ALWAYS know exactly what you have out and EXACTLY what you have available. There is nothing more frustrating for a customer then to find that the rental company doesn't have what was asked for and promised. This will cost you business in the future or can guarantee business as well.
Watch out for kid parties they can somehow go instantly wrong quickly. Just thing folding table and kids crawling underneath them.
Make sure your insurance covers the item and the injury. LOL Self explanatory, some people aren't smart enough to fold a table or a chair.
Have lots of variety and be able to suggest caterers and other types of party products.
Don't offer start-off sales that will hurt future pricing, don't drop your under new management pricing AT ALL!!
NEVER EVER EVER fish off your local volunteer FD. They all talk and they can bury you and your business in about a week with bad feedback. This goes for all organizations of this type they all talk amongst themselves and always make sure bad experiences are passed around quickly.
If I remember more I will let you know. Until then good luck! :hug: