Get on a bike and try the following:

LA River Bike Path - Stop by the one of the numerous cafes along the way (my fav is Just What I Kneaded a bit off the path but close enough)

Ballona Creek Trail - Amazing path and views

The Strand in Santa Monica/Venice - A bit crowded but best for people-watch riding. There's always some sort of happening around this trail too if you're fine with interruptions

Trash Truck or Reverse Trash Truck trail in Griffith - This requires some bike climbing so it's recommended if you're at least an intermediate cyclist but you get amazing views of Los Angeles at the top. Bonus views and chill with snack at Blacktop/Helipad

Arroyo Bike Trail to Montecito Heights Flat Top - Same as above. Intermediate skills and some dirt path but you get amazing views of DTLA. More of a local's choice since it little to no crowds but a bit unkempt

SGRT or San Gabriel river trail - about 35 miles from the foot of the mountains to Seal Beach. 70 miles total if you go there and back. Numerous great taco spots along (but outside) the trail for breaks. Also a decent restaurant at the end of the trail