Asda do lovely prem clothes in much smaller sizes than most high street shops/supermarkets. They even do nappy covers (iirc) so that even if babies are too small/sensitive/need access and vests aren't suitable they can still be "dressed".
Clothes with all the velcro are a pain as the velcro always sticks to the wrong part and they also cling to the nappies! I'm a NICU nurse, I avoid these clothes if at all possible! Obviously if that's what parents choose for their baby I do use them but if they also have "normal" clothes too I use the normal ones.
A good basic hand cream is always useful. Not too many harsh chemicals as they can react with all the hand gel.
Books. There's so many lovely gift books available. I love the Nancy Tillman ones.
Hand knitted cardigans in tiny sizes. They keep the babies warmer than those padded cardigan that come with outfits.
Blankets for kangaroo care. Ask the parents what the storage space is like tho, a lot of blankets are quite bulky and take up a full incubator drawer.
Anything practical to make the parents lives easier- childcare, proper meals, bags of loose change for the car park/coffee machine etc.
As an aside, babies need a certain amount of body fat in order to maintain their temp so weight is definitely a factor when moving from incubator to cot. It differs from hospital to hospital but I'd expect it to happen between 1.6 and 1.8kg if baby was gaining weight and able to maintain their temp in the incubator.