I've just come back from the Fjords (not with P&O) and called in Flam, Olden and Haugesund. In Olden the Briksdal Glacier is a good 45-minute hike each way along a steep path (not a 'slight incline' as on the information board!). You can book independently and if doing the morning tour, there is enough time to then get a bus over to the Loen Skylift. The views are incredible and there are a number of walks you can do. Honestly, I could have spent the entire day doing that.
If you're feeling adventurous, you can book independently to do the Flam railway, a zipline and then cycle back to the port (walking is also an option as is re-joining the train). I haven't been on a bike in decades, but it is mainly downhill (two steep inclines) and allows you to stop and take in the scenery as much as you like.
Haugesund - having read there wasn't much to do I booked a ship tour to Langfoss Waterfall. Unfortunately, the weather hadn't been great prior to the trip so the waterfall, while lovely, wasn't in full flow. 90-minute bus journey each way so not something I'd recommend doing independently.