at 5.5 mo liv was getting tummy time with toys put around her so she could try to reach for them, playing under gymini/baby gyms, soft toys, soft blocks for banging (same for the rings off the ring stacking thing with the pole in the middle). frustration is an ok thing as it motivates them to try to reach the objects etc (so long as thyey are not getting too upset). best place is probably on the floor as much as possible as this is the window where they are working to sit up independently so need practice (put pillows around baby if needed), rolling, and eventually crawling.