When I was in a similar situation we recruited students from other schools in the area. Basically we contacted profs in the language department and advertised that way. I imagine this could be difficult for you during the summer. We were able to get 15-20 students this way. It adds complications particularly in communication, but they're not insurmountable. I would try to get facs from the other schools to accompany as chaperones. Free tickets are thrown in by most tour companies when you have a certain amount of students.
It's not hard to make it a credit course. Just plan out some culture and language learrning activities in advance based on cities and attractions you will visit and circumstances the students will be in (eg. in a host family). These can be as structured or unstructured as you want/need them to be. Sometimes I have given students worksheets or templates in advance, like interview questions, or scavenger hunts. An easy way is to have them keep a log of their trip, with entries either every day or at whatever interval you want. The hassle with this is they may have to do that on paper as opposed to electronically, which makes more work for you and stuff can get lost, etc. A really simple way is to have them write an anticipation reflection before they leave, and a post-reflection after they get back, and submit to you electronically. You can guide these with questions like, what do you expect to see in (country)? Why do you want to visit this country? and post: What surprised you about (country)? describe positive and negative experiences or impressions. Tell 3 new expressions you learned and the context and circumstances in which you learned them. etc. For reflections of this type I like to use an assessment rubric so students know how they will be graded, covering length, how many details you're looking for, etc. you can have them write in English or the target language.
hope that helps.