I voted Isla but it's been so popular for a while. I don't know many little girls but I know 2 Islas (and one Ayla pronounced the same as Isla). It's fine, but overused and a bit uninspiring.
Isobel/Isabel or Isabella looks similar written down. Also has been very popular, but it's gorgeous where Isla just isn't imo.
Elsie to me still has a strong old lady vibe. Yes I realise it's been very popular for a while now. I suppose it's pretty in a way but a young Elsie seems incongruous to me, although obviously there are plenty now. It's also a little cutesy.
Elise as suggested here is much more sophisticated and attractive. Elsa would be OK, or what about Eloise which is gorgeous. Eliza is also lovely.