You can invest with bitcoin alone, I already made a cash out today and made profits. Just holding your BTC is investing because if you buy now and it will grow as you go on because the price trend of it is increasing, well you can apply it to altcoins if you want but always choose wisely.
Bitcoin is already good investment, In this case just buy bitcoin and it can be considered as our investment. Because bitcoin is the long, mid, short term investment for everyone.
Let us take it literally if we buy right now the price of Bitcoin will be 1200$ which is currently high and we can see the price action is taking its time to cross the 1300$. Let me tell you why it is bad to buy right now aside from what I said earlier. Buying right now at the current price is a bad idea because you are not giving yourself a margin of safety in which you need in case the price falls down. Margin of safety is one of the best tools to protect your money from your invesments you should use it.