I figured "school fieldtrip organizers" as the first keyword after seeing your description, and then you can prefix or suffix this phrase with your location/ specific area like "School trip organizers hannover" and so on.

You must be having atleast 1 competitor in your country or in the world (I found many after searching for the above keyword) and then you can see which keywords are they targeting by either checking their source meta or using google keyword planner.

I got many combinations already by doing so (in 1 minute) like:-
1. School trip agents
2. ideas for school field trips
3. school fieldtrip ideas nearby
4. School fun activities organizer
5. School field trip experience

and then so on, and since your business looks highly location specific then do consider adding location name in your targeted keywords.

I can help you more should you ask for, with all this info.

Secondly, all these SEO tactics are for inbound lead generation and I'd suggest you to do consider outbound tactics at the same time since it is relatively new business offering and reaching out to your customers proactively instead of waiting for them to come on site is more important and i believe you know this, I'm just emphasizing.

For my business of App development (https://www.agicent.com/blog/top-10-mobile-app-development-companies-india), I rely on more human-like keyphrases through instead of what google planner might suggest, it is good to measure the volume of search though.

So a combination of tactics and tools would work, instead of just one way.