Due date extended to 31.08.2012 for all assessee (non audit case ) to 31.08.2012 read details here
If balance tax is payable then it must be paid before return filing .It is called self assessment tax.
How to deposit self assessment tax online offline through challan 280 is described in details here.
Penalty on Late filing of Income Tax return after due date Fy 2011-12 Ay 2012-13 read details here
Who can file Sahaj and who Can not file Sahaj is Given here in detail.
Income Tax Return filing mandatory if your Individual/HUF income is more than 10 Lakh in Fy 2011-12.So you have e File your return Read notification .However he may file with or without digital signature.
So if your Income is more than 10 Lakh then you have to e file you income tax return and you can not submit income tax return on paper Form.
Before filing of Income Tax return you should check your Tax deducted deposited and collected through Form 26AS. How to check Form 26AS in three ways is available here