Airtel just introduced a new plan that gives you 180% Bonus for any subscription.
The data plan is not available for all airtel subscribers but if you are eligible for this offer, you will receive a message that looks similar or exactly like this one below.

"FREE 150% Data Bonus, Just For You! Get 1.8GB for just N500 valid for 14 days. Hurry, dial *418*2# now to enjoy this amazing offer everyday for 6 months."

However, if you didn't receive this message simply dial *418*2# and wait for the response to know if you're eligible.
If it fails because of insufficient balance, recharge your phone with #500 and then retry.
If you are eligible, recharge your Airtel line and dial *418*2# to activate the N500 for 1.8GB Plan.

Source NaijaTech