best toys for a 4 month old?

[thisxgirl]

Now my 3.5 month old DS is really paying attention to his mobile, travel toys and the toys hanging off his bouncy chair, I want to get him some rattles and stuff. What other type of things are they keen and able to play with at this stage?

There are so many products on the market, I don't know where to start or which are popular.

I had the Fisher Price Amazing Animals Train recommended to me so that's a Christmas present, when he's almost 6 months.

Thanks!