There are a lot of many Bitcoin sites, which also can sometimes offer good profit. Be smart, when investing in them, even good looking BTC site can be simple scam. From the other hand do not look to every BTC site, as scam 100%. With time you will understand, where you can invest your Bitcoins and make profit. 
For investing I have seen investing in bankroll for gambling sites, investing in mining, and investing in other sites that are profiting off Bitcoin technology, such as exchanges. Any others, and which types would those here recommend as most worthwhile?
I'm trying to invest on bankroll casino to test and see if it's really profitable. Looks the profit is sure. Nothing very profitable, but a small and safe return is possible to get. But as I said, I'm testing it yet.
Another investiments like technology and exchanges, I don't know, never saw any option to invest.