Homemade dj hack ideas?

That sounds really cute! It also depends on your budget because you can do that route and still maintain a pretty decent sound.

If your budget a little bigger, you can rent audio equipment from Long & McQuade to give it a real tech-y feel! Usually when I did audio set-up for touring theatre performances, I had to rent a compatible aux chord, speakers, a mixing board (cables come included). It all can plug into a phone or a laptop with some kind of playback program (i.e. iTunes or Windows Media player). You can ask them at Long & McQuade what you would need for your set-up and they'll happily help!

There's free audio programs like SFX or QLab (with free tutorials online) where you can build playlists and set them up so he just has to hit GO. When set up properly as a playlist, it plays automatically with proper fades to other songs. Once it's started, your cousin can manually adjust the levels on the mixing board in case some songs are louder than others, or cut the music and have announcements over a mic (also rentable and plugs into the board). It gives it more of a professional sound than the start-stop of Spotify playlists.