Just record yourself playing creatively for about an hour.

When done, listen to the entire recording and pick out the best part(s) of what you have recorded. (At least once, if not several times. Maybe listen to it in different places - like while driving, working in the yard, jogging, biking, just relaxing on the couch. Sometimes a location will influence what parts of the recording you will find the best to build upon. And remember there's no reason you can't pull ideas for several songs from a single recording session.)

Once you've picked out your part (or pieces), use the selected music as a base for building songs. Be creative. Try anything. Listen for stuff that makes you say "Yea, that sounds good." (Whether it be a happy, sad, angry, etc kind of music, there's still parts you are looking for that "sound good".)