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).
Gsec futures are called Interest rate futures, they trade on currency derivative segment on NSE. You can find this in our currency margin calculator link.
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: