If you are self-employed and looking for true simplicity, look no further. A Simplified Employee Pension (SEP) is an employer retirement plan that uses IRAs as the funding vehicle. In 2018, you can contribute up to $55,000 ($54,000 in 2017), or 25% of compensation, whichever is less. The actual formula limits your contribution to 20% of your net self-employment income. You can even make your regular IRA contribution to your SEP, in addition to your regular SEP contribution.
SUGGESTION: Your contribution can vary from year to year; you can even skip contributions indefinitely. While this is probably not in your best interest, you also don't have to be bothered with complicated plan documents or annual filing requirements.
IMPORTANT NOTE: As in a regular IRA, you can't borrow from a SEP, nor can you use the assets as collateral for a loan. In addition, the rules against putting collectibles in your IRA also apply to SEPs.