[X] Propose meeting up with Sidhe and going out shopping. Since this is the one plane that actually accepts the weird gold, you'll have enough of it to purchase whatever you want.
-[x] People with similar symptoms or records of same, microorganisms or spores from the diseases, research notes, maybe she could quickly question a doctor?
-[x] Look for toys and shinies, both for yourself+Sidhe (fun effects to learn from), for Nanoha+Mitra+Agneyastra (magical utility items, weapons), and for people you know/families etc (exotic spices, magic knickknacks and tools that tech can't match, exotic fashion items, etc).
-[X] Look for something for Nanoha. Shop for souvenirs for her family and friends together? Her parents have a bakery, so maybe some exotic spices or cooking implements / cookbooks? Or ornamental swords? She would know best.
[X] Ask mom to scan for bills of health or research notes on your last patient. Did the healers try to cure him but failed or merely keep 'the experiment' stable?
-[X] Look for a birthday gift for Sidhe. Something of sentimental value. You have an idea of having a group picture / photo and making that into a gift. So, maybe look for ornamental or enchanted picture frames?
-[X] Do a cursory scan of everything you can find, but don't get too bogged down looking at any one thing. Just buy it and study it later, we have literal tons of gold treasure.
-[X] Ask Mom to take some group photos while on the shopping trip; ideally the whole group (including her avatar and Mitra) should be seen and Sidhe should have a good center-ish position as it's for her birthday present.
[X] Tell Nanoha that your rejection might've been just a teensie bit premature.No, it really wasn't, this is fleeting while you're sure that would've ended badly. Neph even said so.