We took a detour to see Hruanfossar & Barnafoss. It was a little out of the way but worth it.
Glanni waterfall - parked at the golf course. Cost 1000 ISK to park but included bathrooms.
Kolugljufur canyon & waterfall
Reykjafoss - $10 cash to park, has bathrooms. Also has a hot spring you can use with the fee.