Use Groupon for (basically) 2 for 1 restaurant deals.

Most movie theatres have a "cheap ticket Tuesday."

Watch the music videos for today's top hits. I'm not a fan of pop music, but it's entertaining to make fun of really trashy music. And, occasionally, there will be a good song in there.

Build a fort (my personal fav).

Bake together.

As others have mentioned, Eventbrite is a great resource. Facebook will show you local events, too.

Improv and other theatre shows. I went to a hole-in-the-wall theatre a few weeks ago for a fun, interactive show in Chicago. Ten bucks! And you're supporting local artists.

Drunk painting. Haven't tried this but maybe I'll do it as a solo activity...