If you absolutely don't want to use 3rd party software(including open-source), then just simply paste in your wallet address or qr code. Or if you want to be slightly more fancy, add a donate button(on your menu bar or at the bottom of the page) and use a modal pop-up with your wallets address/qrcode when clicked.
Easily the only option that you should've done OP, that's what one of the bitcoin developers do, they put their addresses in their site's donation page, no fuzz. Also, why worry about Google ads? It's a good way to make your site a prominent one, in this day and age where there's billions of websites, you might want to consider being recognized, it's not enough that you're niche.