I especially like the tactic of validating the person you’re talking to in order to communicate that you’re interested in what they’re saying.
E.g. “That’s a really good point” or “That’s really interesting” or “I really like that thought”. And if someone tells a mediocre joke but you want to continue engaging, simply saying “That’s really funny” is a good substitute when the joke isn’t funny enough to actually laugh!
If your sole goal is to make the other person feel like they’re having a great conversation with you, the simplest advice is to make sure they’re talking at least 1.5x as much as you. When someone does the majority of the talking, that person more often walks away feeling like they had a great conversation.