Combining Shimla and Manali in a great travel idea. Here’s a plan that will make your journey more exciting:
Day 1 and 2 in Chail. Nice place to start off with.
Day 3 and 4 in Shimla. Head from Shimla to Manali taking the Shimla - Narkanda - Manali route. You could also take the normal Shimla - Bilaspur-Manali route without any night stoppage, but that would be a boring six-seven-hour drive!
Day 5 - Stop at Narkanda.
Day 6-7 in Kullu. Travel to Kullu on day 6 and visit Manikaran on day 7.
Depending upon the weather conditions, you can enjoy rafting on the way to Manikaran.