Ahhh, the search for a passive, low risk, high return, short term investment never ceases.
This helps explain crypto-fever I guess... No-one wants to believe it doesn't exist. Here are the options you mention.
1) Peer-Peer lending (Majority are 5+ years)
Lately these have been really terrible investments (see thread on it). High risk, low (or negative) returns. I'd be more tempted to try shorting the stock of Lendingtree instead - yet their stock somehow hit an all time high last week with a PE of 143 (!!), despite paying no dividends and all the indications of a ponzi scheme. Go figure.
2) CD - Return is low
Yes. But better than cash and capital guaranteed by the Gov.
3) High interest saving / money market account - (I don't think it is high enough. I'm looking for atleast 7%)
Try Russia, Brazil or India.
4) Individual stocks - Need to be upto date with the market always. I have burnt my hand ones in this area and it is like once bitten twice shy.
Just don't.
5) Bitcoin? - Still studying this market
I guess you could call it a market? It's not what most would call an investment tho. Please don't.
So, what to do?
If you want low/zero risk, and need money back in "1-2 years" then CD in a FDIC bank. (don't keep cash as that will lose money)
If you want some risk, liquid, but pretty safe over 2 years, Vanguard Wellesley Fund [60% Bonds/40% equity]
If you just want a solid guaranteed return, simply pay down your mortgage.