There are a wide variety of resources available online when it comes to creating activities or lessons. Aside from our own collection of activities submitted by and for HAJET members, here as some others that may help you out!
HAJET Activities Database
HAJET offers a database of activities made by fellow HAJET members that you may find useful within your classroom. Lessons in the database are extremely detailed and make it easier to form full lessons around them. In order to use it, you will need to be a member of HAJET. Submissions are still welcome!
ALTWiki is another popular website with a wide assortment of activities submitted by users. The site also features some sections on advice for ALTs in the classroom as well.
EigoGanbare contains an enormous amount of worksheets, audio, and lesson plans to support not only the English classroom in public schools, but after-school clubs as well. Beyond support for students, EigoGanbare also has some support for those studying for the JLPT as well.