I'd recommend a play mat + play fence. play mat can really help in the tummy time It's solid enough for them to walk steadily on compare to a mattress that's soft. Yet it's soft enough to cushion any fall. Play fence really helps when they start rolling or crawling but its better for them to get used to it early.
I got all 3 and I can say play gym is a waste of money. I think what's the best is bumper bed. I never regret investing in a bumper bed.