If you've got a nice rhythm, try doing just down beats on the odd counts:
Then go back to your regular pattern.
Another pattern can be:
The only other different pattern for four counts is to just do up strums on the even counts. This is a little difficult because the pause comes first, but if you're getting bored with the songs you know, this will spice them up:
Fool around with these patterns on the two-chord songs you already know. See where they make the song sound better. Sometimes it's good to change the rhythm at the end of a line or verse. Other times it's good to do an entire verse in one pattern and then do the next in a different pattern.
The main thing is to play slow enough to keep the rhythm while changing chords.
On sheet music, you might see eighth or sixteenths or even thirty-second notes (a dot after a sixteenth note) to try to show different rhythms. That’s a limitation due to the idea of dividing a measure into discrete blocks of time. But time is a continuum. It isn’t a set of standard intervals like the frets on a guitar; it is continuous like the fretless neck of a fiddle. Instead of separate notes, there are an infinite number of notes (most of them wrong). But in time measurement, there is just an infinite variety of rhythms.
Think of all the rhythms you can hear in rock or pop or country. All of those are written as 4/4 (except the waltz ones), but some are played as shuffles, some are slow ballads, some have a hard beat, others feel soft. Another way to think of rhythm is from a dancer’s perspective. The Charleston, the frug, the merenge, and all disco music is 4/4 time. That’s a limitation of the writing and the explaining. I’ve explained a basic strum as 4 even beats, but there’s much more than that.
Feel the rhythm and have fun with it is about all the additional explanation I can provide for adding variety to your strums. And of course, jamming helps.