For our first Opposites study we used pgs 113-121 in the Comprehensive Curriculum of Basic Skills for Preschool.
First, I emptied out two toys bins and designated one for soft toys and one for hard toys. If you are working on reading as well, you can go ahead and put "soft" and "hard" signs on your bins. Once you have your bins, let your kiddo collect things until they're sick of the game! J actually ended up doing this one for a really long time plus she made a pile in between the two bins for items that were both soft and hard! Bonus!
For our second game we basically did a Simon Says with Opposites. Go "in" the bedroom (have them go) then ask "what's the opposite of in?" "Out!" "Out" of the bedroom (have them go out). Keep going: "Under" the table/ "over" the table, find a "big" stuffed animal/ find a "little" stuffed animal, find a "loud" toy/ find a "quiet" toy, stand "up"/ sit "down", "open" the door/ "close" the door, this wash cloth is "dry"/ now get it "wet", turn lights "on"/ turn lights "off", etc.