Dominik_E:
Hi,first of all I am from Austria - so please excuse any spelling/grammar mistakes.
Welcome to the forum! (Not sure I noticed any :D)
I included pull-ups for all the I2C and SPI lines (I read it's good practice to have them even if not needed for SPI), an ICSP header to be able to flash the arduino bootloader and a header to connect to a serial/usb converter.
But the ATmega has internal pullups, so why not use those. When sleeping, it's good practice to set unused pins to inputs and enable pullups. This will minimize current draw and help prevent EMI effects.
You may be interested in a similar datalogger that I designed. I recently entered it in Atmel's "SimplyAVR" contest. Probably the best way to get a quick overview is to go there and watch the short video and read the summary. There are links there to the hardware design and firmware. The contest site doesn't seem to have direct links to individual projects, but if you go to http://app.snapapp.com/simplyavrcontest then click on VOTE in the upper right, my project is currently on the first page in the gallery, bottom row, it's called "Double-A Data Logger". Clicking VOTE just takes you to the gallery, there's no need to vote if you don't want to. Cheers!