Hi Barbara great question this can be hard to do when you learn a song you develop that muscle memory and its hard to deviate from what you learn initially and get so used to playing. Like Skyman says start with very simple things, eg instead of fretting and plucking a note use a hameron and or pull off, slide or bend up to a note. When youre strumming your chords, throw in a random hammer to see how it sounds or even just lift a finger and alter the voicing of the chord. PLay around with it but keep it simple. When transitioning between chords keep your runs to only one, two or three notes (remember when you are doing this it changes the timing ie instead of struming the chord on the beat you are hitting those individual notes). Same with any licks you want to throw in, keep them short and simple. And practice slow, slower that even seems comfortable. When you go too fast at the beginning its easy to get mixed up and frustrated.
You can also always try a different strumming pattern or rythmn and tempo than what the song calls for and what you have already learned. Sure that will change the vibe of the song but remember its ok to put your own twist on any song and you might find something you like….you have to have a very open mind and kind of let yourself go to find that groove.
I also think the better you know the song the way you initially learned it, the easier this becomes. If you can play the song almost without thinking about it, that will free your mind up to add some new things. So my advice is to keep on practicing and drilling in the songs you know, but at the same time continue to get very comfortable with the other skills and techniques. Start slow, keep it simple and with lots of repetition you’ll be putting them together naturally. Be kind to and patient with yourself as you go. As they say, embrace the suck. Remember youre doing something most people only think about.
Last thing…remember how versatile the D chord is, especially on the E string with your middle and baby fingers. Get them moving good and you can add a lot of spice to any song that has a D in it, and as you know there are many.
Again I love your question…really got me thinking about this …of course this is what we are all striving for…to put it all together and make our songs sound as interesting as possible. Im in no way an expert but heres a few thoughts. Hope they might be helpful to you. Rock on sister