When I'm manic it's craft supplies. Scrapbooking, mostly, and expensive electronics (Laptop, new computer, etc). Now that I'm living with my sister, my hypomania shows itself in buying things for the kids. Kids' craft supplies, fuzzy purple pillows, camo sheets, things that will make them happy. (I have enough adult craft supplies to last me til Armageddon). Also being on disability, with no credit (except on computer equipment) helps keep me a little bit in check on how much I spend.
I guess everyone has to find their own manic spending items, and watch out for them. Lately, I've been wanting to buy a new computer.....