Welcome to the Guild Palanolho! I think that at the scale you are using, the river is really the only one that is really needing love. The forests and mountains both are nice and effective.
Now, on to the river. A lot really depends on what software you have available to work with and how much time you have to spend.
If you throw up the base image hex with the grass alone (no river stuff at all, I am sure I or someone else could throw together a few examples over the next day or so. Of course, best if we can help teach you though so you can make modifications later (such as rivers with different shapes/blends, etc.)
One large help if you really want to redesign the mountains would be to check out the tutorial by Andy Law (Hapimeses) Simple Mountains [video, Photoshop/GIMP/Painter/etc.]
However, with that said, if you go with that style, you should really think about switching all the tiles to ISO style also. If you want to stay with the top down approach, I don't think you have much choice as if you bring up the scale to make them look taller, you also make the wider and they crowd out of the space alloted. of course, if the mountains are that small, then the rivers at the scale you posted would be freaking hughnormous, so you have to watch for that also...
Enough rattling off... perhaps something I said helped point you in the right direction... or gives you some ideas for further clarifying questions.