It all depends on a lot of factors such as "Who are you", "What are your talents","How experienced you are" etc.
I've faced the same problem from a different angle - I had an empty office (fired the whole crew) and had some budget for a couple of mid-level designers and maybe a PSD-to-HTML guy.
Then I decided to let my mates in and build a Call-Center (I had no idea about this field but my office was empty and they needed an office), started to see some returns lately so sometimes you just need to find someone who knows what to do with $15k and become a partner (an active investor).
If that's not what you would like to do with your money i'd suggest starting a small outsourcing company (it depends a lot on the place you live in cause with high-salary zones this business will be a disaster for a newbiew). You can find jobs for your team on freelance sites like UpWork, Freelancer etc...
If managing and dealing with people in a physical place isn't an option for you I would suggest one of the following:
1) Burn it all on Media Buying/Facebook ads. You can easily lose everything and then cry a year about it OR you can get an understanding on how things work there (Invest->Test->Invest->Test....) and get a couple of successful campaigns running and making you happy with a nice ROI%.
2) Create a product. This is a tricky one since you'll need to: A) Think of a great Idea B) Market research & Discover your ideal client C) Look for the perfect team D) Develop your product E) Launch & Promote.
If A,B,C,D can be established by nearly anyone with some motivation E is a pain in the ass.
If it's a Software, eBook or a Script you'll need create affiliate pages, contests, contact affiliates, make them believe that your product is the best to promote and that your landing has the highest EPC's etc..
If it's a mobile app/game you'll need to promote it yourself (same as investing the whole money into media buys) AND to turn to CPA companies to Pay them Per Install which can lead to loss of money without a proper business model, retention, UX or any of the other 10+ factors.
That's it, those are my 2 cents. Hope you'll do well.
