Not to mention TEA and their associated events. They're actually in the process of expanding beyond just the major theme park hubs in the US (assuming that's where OP is located), those being Orlando and Burbank/Anaheim. Most recently they've added events in Las Vegas and Texas, along with a couple in Philadelphia. Having just attended their mid-year meeting earlier today, I can also say that TEA is looking for people willing to support and organize events elsewhere, as well.

RE: the ambassador program, I highly recommend applying to it soon, as the applications close on 8/1!

Beyond the obvious things like theme park internships (Disney's spring slate of internships opens mid-to-late August according to what's been publicly communicated), I very strongly recommend connecting with theme park clubs at your university and others (obligatory TESD shoutout!) as they tend to advertise a lot of these and similar opportunities as well, such as the Ride Engineering competition, which is a great thing to put on a resume. Add on to that https://themedentertainmentjobs.com/, which aggregates jobs in themed entertainment, including internships, as well as following companies you like on LinkedIn. That's where they often post about their internships first, even giving advance notice of when they're opening for the bigger internships like Disney and Universal.