The simplest and most cost effective I’ve found are the Bose Companion 2 series III, $89 from Amazon or Best Buy. Everything you need, including wires, is in the box. They plug into AC for power and into the headphone jack or external speaker jack on your computer. They have a strong bass, powerful midrange, and a built-in amplifier with volume control. Translation: they will play loud enough for playing along on an online session. I wouldn’t call them audiophile, but they get the job done. You do need to make sure your computer has either a headphone or external speaker jack sized for a mini-plug.
Other under $100 wired speaker options Presonus Eris E3.5 and Mackie CR3-X at Sweetwater. You'll need to buy the needed wires separately; Sweetwater will help with that selection. There are other speakers to choose from, but I can personally recommend the ones I've listed.
USB and Bluetooth speakers
If your computer has only USB plug-ins, there are USB speakers but they tend to be small and powered via the computer’s USB. Bluetooth speakers are another option and the Anker Soundcore Motion will play loud enough and will work with iPads and phones. If you go this route, make sure they will play loud enough and with enough range from bass to treble for play-along. Finally, it may be possible to use smart speakers you already have, such as the larger Alexa if you can link them to your zoom and playback source.
The other option, and I sometimes do this for play-along sessions, is to use headphones. I mean old fashioned “on-ear” headphones, that rest on or around the ears, not in-ear earbud types. In-ear headphones can damage your hearing when set at the necessary volume. I recommend open-back headphones for play-along. They allow the sound from your recorder to reach your ears. Sealed on-ear headphones won’t work because they will block the sound of your own playing.
Here are my recommendations for open-back on-ear headphones (although I repeat that speakers are the better option):