Costumes
A good way to not spend a ton of money/have clutter in your house is to co-opt your normal clothing for Halloween costumes. Or to at least buy clothes that could be used for a Halloween costume. You got Mary Janes and a black dress with a white collar? Congrats, you’re Wednesday Addams. Red/white striped long-sleeve and a beanie and jeans? Found you, Waldo. It gives you a chance to be creative and more versatile with your everyday wardrobe. Also, I see you procrastinators. Of course, if you do buy “normal clothes” with the intent to wear them as a costume later down the road, make sure you do actually wear those clothes on a normal basis. If you’re being particularly lazy that Halloween, just wear orange and black and call it a day.
There’s also thrift stores and pilfering your friend’s old Halloween costumes. Clothing swaps are popular now - why not also swap old Halloween costumes since most people don’t wear their costumes more than once? You could also be a rebel and just wear the same costume every year. Make it your thing. Dress like a witch or a wizard every year and change out some small accessories and make people guess what you’ve changed.
Parties
Making Halloween parties green is mostly just about reducing waste. Make sure you get an accurate headcount and buy just enough food to cover that. If you have more, line up people who would gladly take the leftovers.