I've been doing tummy time with my baby and I have a similar experience of them being super interested in toys during these sessions! My baby loves it when I place a toy just out of reach, it encourages them to stretch and move towards it which helps build those strong neck muscles. It's also helpful to mix up the routine by placing toys at different heights or angles so they have to think and problem solve on how to get to them.
I've also found that introducing small textures like soft books with different fabrics or a toy with a rough surface can add variety and keep things interesting for baby. You might have already tried this, but we also used to place a toy just above baby's forehead as they look up, it helps strengthen those neck muscles too and is great for visual stimulation. Would love to hear any other experiences or ideas you may have!