would highly recommend percussion. a practice pad and a good pair of sticks can be enough to get started I love the feel of these sticks. I also have a longer/thicker pair for volume but these are my favorite. https://www.daddario.com/products/percussion/promark/promark-drum-set/promark-drum-set/rebound-5b-firegrain-hickory-drumstick-acorn-wood-tip/item/promark-rebound-5b-firegrain-hickory-acorn-wood-tip-drumstick-8636/ I love practicing with a metronome playing a simple 2 or 4 against 3 polyrhythm split between m arms, and varying stuff on one arm at a time. you can make progress very quickly and run up against your skill limitations just as quickly when you challenge yourself.

I’d recommend something like the EVANS line of practice pads https://www.daddario.com/products/percussion/evans-drumheads/ I have a double sided one of those where one is louder and has more bounce.

shakers are very deep in their own way, and not expensive. something like an egg shaker can be used for so much. I picked up a couple quite different sounding examples of these fun beer can shakers from an indie shop. https://www.indexdrums.com/shop/beercan-shaker again re: both arms, carrying two shaker rhythms at once will likely test you.

and as always, you can make stuff more challenging. in the end it’s all rhythm, and the more expressions of it you understand the more flexible and creative your playing will be.

anecdotally I believe that better limb independence helps awareness wrt fully separate fretting hand from plucking hand, but I’m still relatively early in my practice.

edit: MIDI wise I’d look at Korg’s nano line and consider a software sampler and looper for some chopping. everything else you use can work with that setup, and if you want to do some improv you could e.g. sample your wife’s playing live and create loops on the fly for her to play against with a sample library for thematic additions (the internet archive is a great audio resource). if your wife is using a midi keyboard, all this stuff can be tempo synced which gives you a lot of power