For getting used to strumming open chords and making chord changes faster, Hey Joe (the most famous Hendrix version is a fine tempo) was what I found most useful when I was starting. Since it uses (or at least can be boiled down to) C, G, D, A and E open major chords, if you can get those transitions down then you're set for 99% of popular rock songs.
To break up the dad-rock monotony, I'd like to suggest you check out some of the simpler mid-late 80s rock and hair metal. They're all standard open major and minor chords for the verses, power chords all the way through the choruses, a reptitive pentatonic ''solo'' and back to some powerchords until you fade out. Nothing that will stress your hands but the slightly faster tempos will help you focus on making the chord changes quicker. Bon Jovi's Wanted Dead Or Alive and Guns N' Roses version of Knockin On Heaven's Door are two perennial favourites that pretty much every 6-stringer
will learn at some point in their first year or two of playing.
If you really want to get your right hand rhythm down, give some early Prince a go. When You Were Mine, I Wanna Be Your Lover and Sometimes It Snows In April use very simple chord shapes but they're not ones you'll find in many other songs (so you're expanding your chord vocabulary) and the strumming is deceptively more complex and harder than it first sounds in all three songs. Any intermediate player will nail them first time, but 6-string beginners need to concentrate and that concentration is what will really make your hands improve.
And if you're willing to break with this board's traditions and try something that was recorded within the last twenty years (heresey! How dare I suggest such a thing!) then may I recommend you give
Lights' Cactus In The Valley a try. It uses only a couple of simple chord shapes but in positions you don't normally see them and the slight swing of the country-esque rhythm can be a strumming challenge for those whose strumming hand isn't quite moving freely yet. In fact a lot of modern pop-country is good exercise for your strumming hand.
