I think books only work if they are good quality and work with the theme, say a mog book with a cuddly cat toy for a 2nd birthday, or a Lego book that comes with a figure for a 6th birthday.
Seeds, my ds didn't actually mind getting them when he was younger, we never remembered to plant them though.
Plastic tat, kids might like for a minute, but the impact on the planet is just too much.
It varies a lot by age, but I think things that work with a theme, are as reuseable/recylable/lest plastic waste as possible are best- nothing is perfect though.
It massively helps if you can sew, I can't but thankfully my dm is amazing. So we've been able to give out personalised super hero masks & capes, and personalised fabric bags before.
Shop bought things that went down well, thay we've either given or my ds has received:
Metal beach bucket & spade, with sweets in the bucket.
Mini lego figures
Waboba gravity ball
Frisbee
Winter gloves & Hat set- matched the theme and I think they had put sweets inside the hat
Mini torch
Notepad & fun pens, smelly ones/metallic etc...
Hot chocolate set- mug with hot choc sachet, marshmallows & biscuits (that was a secondary aged party)