Looks like you've still got some airwires -- Might want to connect those up!
You might simplify routing by putting U6 above U5, or might not. I don't know hat U5 or U6 are, but it's probably not a big deal.
You have a lot of traces that are much smaller than they need to be, making them wider will reduce inductance and make the board more manufacturable at no cost.
I don't know too much about via stitching, but it seems like some vias under the AtMega might be a good idea. Also add a decoupling cap for the AtMega.
Most importantly, you NEED a big cap for that SIM900!
EDIT: just realized that U10 is an LM2596. I think this switcher layout probably needs work, but I'll leave that up to the switcher experts on this forum