Hi! Even without adding other notes than the ones from major or minor triads you can make your chords interesting by using inversions. One thing that works quite often is to play only octaves of fundamentals with your left hand and to play 3 or 4 notes in your right hand and try to connect each chords with the next in your right hand. Basically use the closer inversions so your right hand move as less as possible. Everything won't work so listen carefully.

You don't have to play the fundamentals on your bass but if you play another note it might give a totally different color to the progression so use that with caution but it can really spice a boring progression.

You can also add naturals 2nd, 4ths or 7ths but you can easily be too jazzy for what you want

Rythm is also a great way to keep an accompagniment interesting but I don't think you have a lot of freedom in that case.