Hey Maaike,

well, Lofoten are amazing and it fits nice in a round trip with Interrail.

As you wanna go home via Kristiansand, it would be best to start via Stockholm and then nighttrain to Narvik. From Narvik you have buses to the Lofoten. I would focus on the Western parts of the Lofoten - the Eastern parts are not so spectacular. There is a “Travel Pass Nordland” which offers quite affordable travel on the Lofoten, you can buy it via the app “Billett Nordland”. It is also valid on most ferries to Bodo as far as I know (at least my brother could use the Bodo-Moskenes and Svolvaer-Bodo ferries without problems). On Lofoten you have a lot of nice hikes and small villages to do/visit. To make it a round trip, would suggest to take the Moskenes-Bodo ferry and then from there continue to Trondheim (also worth a stopover).
As you didn’t tell about how much time you have for the trip, difficult to suggest a full itinerary.

But some more suggestions (alternatives):

• go to Ystad and take a ferry to Bornholm (Danish island), there are some nice beaches
• cities: Stockholm, Göteborg, Trondheim, Oslo, Bergen
• Inlandsbanan Östersund → Gällivare long day trip, but pretty cool and there are 2 food stops on the way. In Gällivare hike to the top of the nearby “mountain”, offers views to Norway and nice midnight sun views
• Flam railway (not included in Interrail, but you get a discount)
• Oslo-Bergen railway line

Preikestolen I find overrated - the hike there is super boring and it is like a highway of unexperienced hikers. I was there in 2014 and suppose with the rise of instagram it didn’t get better but even worse.

For many trains you will need reservations, don’t forget to make them - otherwise they might throw you out at the next stop… some informations on the internet are not true anymore - in Norway you need now also for long-distance trains during the day a reservation (not only for night trains!)

About accommodation: it is expensive . Last summer there was e.g. no hostel in Oslo (or it was fully booked all the time?), and even hostels are expensive. But as you are traveling with your boyfriend, it isn’t going to be that bad - often a private room can be similar or only slightly more expensive than two beds in a hostel…. For hostels: most hostels in Scandinavia charge extra for bedsheets… usually around 7€! But you can bring your own one.
On Lofoten etc. I’d suggest to bring your own tent and from time to time stay in a camping area so that you can use a fresh shower.