A. You can open a Solo 401(k), but if you already contribute fully to your full time job's 401(k), Solo 401(k) doesn't help you maximize your benefits - Solo 401(k)'s contribution is lumped with your other 401(k) to meet the annual $17,500 limit.
SEP IRA will be the best option for you in this case, and it is the best kept secret for people with a side business. SEP IRA is very similar to traditional IRA or Roth IRA or 401(k), with two key advantages:
a) No income limitation.
You can contribute up to 25% of net profit from your side business, subject to the limit below.
b) Elevated contribution limit.
You can contribute up to $52,000 in 2014 (vs. $17,500 for 401K) which means your side business could earn up to $260,000 and you can still fund a SEP IRA.
Of course, SEP IRA also comes with the as expected tax-deferral benefits, and low maintenance cost (you can open at any brokerage houses).
You have till April 15 the following year to contribute to your prior year's SEP IRA (this is especially helpful if you are unsure about your side business' earnings, you can wait until your Schedule C is done, then determine how much you can fund the SEP IRA).
You can visit our previous SEP IRA FAQ's, also can check SIMPLE IRA FAQ's, but in your situation, SEP IRA will be the optimal solution.