I guess there is nothing more guaranteed than government bonds/secs. To buy this directly you need to open a gilt account with one of the banks who offer this facility (I know IDBI bank does). But this is quite a painful process, I guess the best bet is to buy gilt funds (mutual funds that invest into government securities).
You can now easily invest in G-Secs with your Zerodha account. The exchanges have enabled a non-competitive mechanism for retail investors to invest in long-dated bonds and T-bills. The bidding window is open from Monday to Tuesday for T-bills and from Tuesday to Wednesday for bonds. You can invest in multiples of 10,000. Check this post for more: