Your iphone has Bluetooth, doesn’t it? (It may or may not have aptX HD, but whatever.)
So you could use any portable Bluetooth speakers. Or, if you already have a sound system but not Bluetooth, get a bluetooth dongle with an audio jack output.
But if you’re techno-feeble, you might get frustrated by the times you’ll have to re-link the Bluetooth. Maybe you’d rather just plug a good portable speaker into the headphone jack.
But you’re more techno than I am. I’m still using iTunes and loading my mp3s of Old Time Radio (and the Kingston Trio) onto my iPhone.
Ultimate Ears also makes some pretty great little speakers that are drop resistant, water proof, and last 8+ hours on a charge. I bought SWMBO the WonderBoom 3 and it has worked out really well. Super easy to set up as well with the provided instructions.
The Apple lighting headphone dongle is around $8.00.
I have these BlueTooth speakers in my shower. They sound decent, are very waterproof (hence, shower), and the app makes them easy to pair and set up. You can also just get a single speaker instead of the pair.