Free time is great when traveling alone and less useful when traveling with friends IMHO. Alone I'd pick 1 activity and 1 restaurant per day and have a loose idea of other options.
I've gotten a lot of mileage (pun intended) out of walking from activity to activity. Relatively random walking is a surprisingly high return on investment, but only traveling alone.
Google Maps "Attractions" is surprisingly good. CultureTrip, Reddit (frequently location based subreddits will have a sticky thread about highlights), AtlasObscura, and google were also great for research. "Unique things to do in ..." is a great search term, "unique foods in ..." is also great. There are also several activity websites which are good to page through.
Hostels are pretty great and incredibly easy to manage. Hostelworld is a great app.
Rome2Rio is a fantastic app for route planning.
I try to make a list of things like the best bakery, a top tier restaurant, a trendy restaurant, the best dessert, a trip through a grocery store, a trip through a fruit stand, and things like that.
I think when booking travel hostels are immensely helpful for figuring out what to do. It's most important to pre-plan super popular or limited quantity activities ahead of time, but other than that, often times you can just show up at a hostel with nothing planned and page through the hostels activity book/ask the staff what's cool.