I taught an English conversation class for a couple of years in Japan while I was a student and my biggest takeaway is that the classes should be tailored to the individual students as much as possible.
When you first meet your students, spend the whole first class chatting with them, trying to gauge their level and, most importantly, finding out what interests them and why they want to learn. If at all possible, group together students with similar interests and ability and keep the classes as small as you can. In an hour’s class, you should be trying to give each student an equal amount of contact with you (it’s really easy to let more fluent students dominate but that can be hugely frustrating for people without as much ability) and if you have more than five students together, they’re going to get ten minutes or less face to face conversation.
Plan your lessons beforehand so that the students feel like they have learnt something and didn’t just waste an hour chatting about nothing. After a while, I had a bunch of lessons I had trialled and adjusted with different students that I could break out and use when no one had anything they wanted to talk about or that they wanted me to explain.
My favourite of those was prepositions, although Spanish prepositions are probably much more similar to English than Japanese ones and this might not work for native Spanish speakers:
First five to ten minutes were chatting, getting them to tell you about their day/week, to try and get them to feel relaxed about speaking in English. Remember to ask “why?” and “how?” in order to make them form more complex sentences than just “I went shopping. It was fun.”
Next, spend ten minutes or so quizzing them on various prepositions (if your Spanish is good, explain to them the differences in usage between the equivalent prepositions in English and Spanish). Write them down, or if your students want practice, get them to write the words down for you.
Once you have enough prepositions in a list, draw a picture. I am terrible at drawing, which helps break the ice a bit when you ask “so, where is the cat in this picture?” and they say “ohhh, I thought that was a car!” Get them to explain the relationship between objects in progressively more complex sentences.
Spend ten minutes or so drawing pictures for them, then get them to draw pictures for each other. If you keep the conversation flowing all the time, this can be remarkably fun and it’s a good way to jump between comprehension and application of language.
Spend whatever time you have left once the drawing runs out of steam answering questions about things that have confused them so far and asking them quickfire questions about things in the room. Make them ask each other about other objects, so they can practice question composition. Ask them about their plans for the next week as a warm down to the lesson.
Like I say, most of the students I taught had little to no English and I don’t imagine that many Spanish speakers would need you to explain “on, in, under” in that much depth but the reason I wrote all that out is to show how you can take something simple, drill it, get your students to practice it, then get your students talking to each other, and turn the whole thing into a series of activities that flow into each other, to keep the pace of the lesson up and make sure there are no awkward pauses.
It’s pretty terrifying to speak to a bunch of people you don’t know in your native language, let alone in your second. Keeping things structured helps to make the whole process less scary and helps your students to feel like they didn’t just waste an hour talking about TV.
As a side note, I absolutely hated free conversation with other students when I was studying Japanese; it’s impossible to tell whether or not the other person is making mistakes and it feels like a waste of everyone’s time. They’re paying you money to talk with you and learn how a native speaker uses English, not to pick up bad habits off each other. Same goes for presentations; it turns into a rote memorisation exercise and there’s too much to pick apart in even a two or three minute monologue to make it useful for a language learner.