We went away (by car) when Nina was 4 months old. All we really took was a few sets of clothes, both summer clothes & winter clothes, and a couple of her favourite toys. At such a young age they ammused by anything. It's a good idea to only take nappies for the first day or so, then buy the rest, as that alone takes up a lot of room!!
If you feel you didn't take enough while there, you can always go on a shopping spree