I mix things up all the time. Each day I have a different routine and I'm constantly adding new things. I've been running, for example, and now I run to a playground and do some strength training (pull-ups, etc.) and run back.
When it comes to weight training I have one rule: If I am not terribly sore two days afterward then I must add more weight.
Listen to your body

if something is getting easier then switch it up!