Buy Blackberry, have their software team and Nokia's work on messenger, social, navigation, and music apps.
Sponsor more free developer workshops and update the Marketplace to have a third of the main page on all versions of Windows be just featured apps. Have a team just dedicated to finding the best active developers making the greatest apps (no matter how obscure, which is why you need a dedicated team) and pay them awards when featuring them - it's a double incentive to succeed.
Take BES/BIS and integrate it as a service into Windows Phone 8 in a thinner, titanium or carbon fiber-body Lumia 1020 with a Blackberry-style keyboard that can slide out vertically or horizontally, bundle it with all of the 365 services for 2 year contracts, call it the Windows Black Phone, and then put $500 million into marketing it and putting it for free into the hands of certain celebrities and CEOs (or even paying them to use it).
I would make color anodized aluminum Xbox phone, pay HTC to manufacture it, develop a bunch of gaming related accessories for it, and have Microsoft Studios skin the Windows Phone interface and put more Xbox Live features in, and give away a year of Xbox Live Gold and a year of Xbox Music subscription with it. Pay a few high profile gaming companies like Bethesda and Valve to develop exclusive mobile games. Find some way to get Valve on board to do Steam for mobile. Pay another $500 million in marketing and sponsor gaming events.
Ramp up production of the low-cost Lumias and bring cost down by economies of scale, even if they have to contract 5 different international manufacturers to do it (approach Huawei, ZTE, LG, Lenovo, etc) and pay billions. If Microsoft can dominate the developing markets initially, they can own it for a few years. Kill the Asha line. It's just going to make the company compete against itself when a market is ready to "graduate" from Symbian feature phones to full smartphones.
Buy B&N for their college bookstores, partner with publishers for best deals on ebook versions of textbooks, and bundle those e-textbooks with Surface RTs (hardware at zero profit, but a little profit on the ebooks) and sell them in those campus bookstores. Make Surface Pros available at steep discount to enrolled and verified students and normal price to everyone else. Give discounts on all services and other devices in the Microsoft ecosystem. Invest heavily in 8" Surfaces (purely RT without desktop - no Windows branding). The problem with RT isn't that it isn't Windows, it's that it confuses people who think it is. No one confuses iOS and OS X because the iOS name isn't very visible in iPads and iPhones, and there isn't a desktop interface.