I agree with all-in-one singlets as well. On hot days my son usually just wore a nappy and maybe a singlet. Lightweight shorts/t-shirts were worn when we were going out. A rule of thumb i read when i was pregnant with No 1 was to dress baby in whatever i was wearing, plus one layer. So if i was wearing a t-shirt and skirt, he should wear a singlet, t-shirt and shorts. You can get quite lightweight babygros as well.