I will personally suggest don't waste your time staying in Shimla only. 1 day and 1 night are more than enough to explore Shimla all together. Additionally you can go to kufri as well. But it will be much better if you take a cab from Shimla to a place called Narkanda. it's around 60km from there. There you will experience the peaceful and calm mountains. There are quite a lot of adventure activities there as well one for example is Trek to Hatu Peak. It is an easy trek and you can even stay in camps close to Hatu Peak. you will defenately love it.
Just because you are comfortable with solang valley.. so a better option can be "tirthan Valley" as well. because it's a perfect blend of heritage sites and peaceful calm Meadows. if you are interested into trek you can also check "Pin Valley national park" there.
1 day for Shimla (that includes mall road,church,lakkar bazzar , Indian institute of advanced studies and you can also adjust Chadwick falls) 2nd day for kufri(adventure and sightseeing) and jakhu temple
3rd day for narkanda hattu peak
3 days are sufficient for shimla. ???? have a safe trip!