What equipment do you have?
Small-group training is one of my specialties, and although I accept clients of all ages and fitness levels, I tend to get a lot of Boomers who fit into the fitness category you describe – good cardio and general health, looking for more strength and function for daily life.
I do a lot of partner work. My classes are usually 4-6 people, so we’ll start with one pairing and then switch partners several times to encourage community / social / friendship / retention / fun.
1) Lebert Buddy Strap partner bicep / tricep
2) Partner-resisted obliques using CrossCore 180 https://www.youtube.com/watch?v=yFAy9FN7iV4&list=UUMRkhpJ5n03DDlfrxQM58k…
3) Partner-resisted obliques / chest / bicep using bodyweight only https://www.youtube.com/watch?v=h5rrmJVY-0E&list=UUMRkhpJ5n03DDlfrxQM58k…
Partnering isn’t the only thing I do, but that might give you some ideas.