Last fall when I studied abroad I went to Design Festa and I had such a good time that I thought I would go again. Design Festa is a giant art fair where artists have their own individual booths to sell and/or display their work; several thousand artists come each season and it is a huge event that usually lasts one weekend. Other than artwork, there is a stage where local artists come to perform. It is held at Tokyo Big Sight on Odaiba, a manmade Island in Tokyo. You can purchase tickets beforehand at a convenience store for 800¥ or you can buy the day of for 1,000¥.
Summer Design Festa was definitely a lot smaller than last fall's. This event only took up two exhibition halls, but in the fall there were about four exhibition halls on two floors full. That being said, two exhibition halls was still a lot of booths, however there was a lot more sitting and walking room so it didn't feel as crowded.
There was also less weird things this time, and when I mean "weird," I mean disturbing art and people in interesting costumes. There was some NSFW stuff and some people in funny costumes, but overall it felt tame (or I have gotten used to the weirdness).
I didn't spend as much money as I thought I would. I mostly focused my efforts on finding cute bread/sweets keychains and decals because I find them adorable lately. I personally don't even like to eat bread that much, but I find miniature bread items so cute. I bought some bread candles, a bread pin, and a cat bread sandwich key chain.
Even if you are not that interested in art, I would recommend to go to Design Festa to experience Japanese pop culture. I think there is something for everyone there (i.e. anime merchandise, kawaii characters, traditional Japanese items, etc.) and you can spend hours just going up and down the booths. You can also practice your Japanese by speaking to the artists, but I didn't talk that much today. There was also some unconventional booths that did things like perform acting skits, give wound body makeup, and put on a timed art competition. If I lived in Japan I would come every season if I could!