That route is quite good for the time you have. An alternative is to take the A591 Ambleside to Keswick road and then go out on the same route to the junction with the A595. From there follow this road towards Carlisle and then the A689 to your destination.
This route will afford some excellent views of the mid belt fells including Helm Crag at Grasmere and Helvellyn beyond Thirlmere. From Keswick you can get great views of Cat Bells, Maiden Moor and as far as Haystacks. In the other direction you can see Latrigg and out to Blencathra.
Further along north of Keswick you will get views of Dodd and the Skiddaw massif. As you go past, believe it or not but you can still look back south and see Haystacks. You will also pass Binsey.
However, my guess is that this route is probably going to be longer than your 1.5 hrs.
So - if you want to cut across sooner then coming out of Keswick take the A66 eastbound, you will still pass Blencathra and Great Mell on the other side. From there take the M6 north and cut off as required to Brampton. There are plenty of lay-bys on the A66 if you want to stop and take some photos.
It's a shame you don't have longer for the scenic drive because there are some nicer drives than this - however, Lake District roads can get quite small and progress can be slow.