I've had some great externships and some kind of "meh" externships and here's what I've found is the difference at the ones where I come out of it going "wow, that was great, I learned so much!"
-Engagement. I'm very good at standing there silently when needed, but I really appreciate when the doctors or techs include me in the conversation. I'm there to learn! There's one private practice I spend time at regularly because it's near my parents' place, and I get a ton out of being there because the doctors explain to me what they're doing, or ask me how I would approach something. Sometimes I haven't known the answer and they've made that okay - they just tell me where to find the answer, or tell it to me and then expect me to remember it. I don't mind being asked questions in front of clients when I'm there because they understand that I'm learning, but the vets also expect me to know my stuff.
-Clear expectations. I've shadowed some vets who don't want students to touch anything, even if they're just grabbing some gauze out of the truck. I've shadowed some vets who had me jump in and do things. I'm fine with either end of the spectrum, but if I don't know what you want, I'm going to err toward not doing anything, just because I've been reprimanded before. And if you do have me do stuff, I really appreciate feedback!!
Getting hands-on experience is amazing when it happens, but I recognize it doesn't always. It's just an extra treat when it does.