It's hard to say because everyones so different and approach IVF so differently.
Some would love a load of chocolate and comfort items for example, whilst others will be very strictly eating clean or following IVF diets. Sometimes you don't know what you'll be like until it all gets started.
For me personally, bath items or smellies wouldn't have been ideal, as at some points of the process you've advised not to take hot baths or use perfumed/scented products prior to theatre, no nail polish etc. The last thing I felt like doing was pampering myself whilst it was all going on, but others will differ and it would have provided a nice distraction.
Honestly, just a card with a personal and meaningful note inside would have been lovely for me and perhaps a nice water bottle. (Although advice on hot water bottles varies from clinic to clinic. Some recommend hot water bottles to help with follicle growth, others suggest avoiding it)
I've seen some things on Etsy previously like comfy socks that say "IVF got this" or IVF milestone cards or diaries to help track the journey. They might be nice touches? Socks i'd have always enjoyed as you have your legs up in stirrups so often having covered, cosy feet would be one less thing to worry about 
Or some people swear by fertility acupuncture prior to and after treatment. I'm unsure on the cost, but a voucher for that could be something to consider.
Sorry, I know over all that's not very helpful but it pretty much sums up the IVF process unfortunately- no hard and fast rules.
Best of luck to your step daughter, I hope everything goes well and they get their miracle.