I remember at that stage having these Fisher Price Toys that if they pushed a button, or slid a lever, a barnyard animal would pop up. And make a little squeak. And then there was an old Fisher Price toy I had where the 'people' were actually balls. Lots of things to try to push down and as they get a bit older (maybe 8 months) they can use the balls more.
They were handed down to me - I'm not even sure if they make toys like that any more - but they were excellent because it was a bigger toy so would occupy them for a while. It took a while for them to master it (maybe months) but that's when I would show it to them and there was always something they could work.
Kind of like those old toys we hung on the sides of cribs back in the day. Where they could boing a lever and it made a noise, and you could spin a circle and push something to make a ding ...
I may be dating myself now.
If I can find a modern variation, I'll post the link. But those were life savers to me. But like you, my older toddlers were the biggest entertainment. Wish I'd had fish. Never thought of that!
ETA: Couldn't find what I was looking for. They must no longer exist.
But I did have one of these and it was well loved. Mind you the music drove me nuts and I still remember it. But this provided a lot of fun.
http://www.walmart.com/ip/Fisher-Price-Amazing-Animals-Sp...