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.