Yes, but I still think that many of those valid attempts also have ancillary benefits. If I have to drive over the the rental to fix something, and I have to drive right by Costco on the way home, is it tax fraud to stop and shop even though I'm not adding any additional mileage to my trip? This is a tangible benefit I have derived from the rental, by writing off the mileage for a Costco trip that I would have otherwise taken for personal reasons.
The IRS has some definitions on travel expenses (for example, if you go to a convention in another city, but decided to add on a day or two for pleasure before/after the trip) that one can use as a guideline and comparable basis. Also, the IRS permits you to use a mileage rate for business-related travel from your principal location of work to a 'jobsite' (which your rental could probably count as).
If you have a side business, you can still contribute some to a SEP IRA or Solo 401k even if you max out a work 401k.
What are the requirements and restrictions on these? Can you contribute 100% of the revenue for the business? Of the profits? More?
The IRS publications do a fairly decent job on explaining the options and requirements. Set aside an hour or two and look at the IRS publications/forms on Small Businesses and also on Retirement Plans.
A quick and dirty summary:
Solo 401ks offer the best bang-for-the-buck if you will only have self-employed income of up to $20k-$30k, since you can divert a much higher % of your income (I believe up to 100%, max of $17k, but verify). However, they require some fees/paperwork to give to your broker to setup and maintain it (maybe $150 in fees/year, plus some forms). Also, your max limit for a solo 401k is limited, and I don't recall the max limits if you simultaneously participate in a 401k with your full-time employer.
A SEP IRA is the easiest to set up, and typically doesn't carry any additional account maintenance costs w/ your broker (over and above standard account minimums and standard low balance fees). Also, you are allowed to put up to 20% of your NET self employed income (or 25% of your gross income, before self-employment tax, I believe - basically the same net contribution number, just a different percentage of a different number). The max annual SEP IRA contribution is roughly $46,000 of contributions (based on self-employed income of around $220,000). Exact numbers are probably dated by a few years, but you get the point.
Bottom line: If you will have relatively lower self-employed income for several years, it could be worth it to max out your Solo 401k for several years, build up the stash, and pay the $100 or so annual maintenance fees (for a decently-sized account). However, don't forget that if you aren't going to have anymore self employed income contributions to a solo 401k or SEP IRA, you can roll those over into a traditional IRA or rollover401k....so if/when you aren't going to have any more self employed income retirement contributions, you should be able to roll over that solo 401k with those pesky fees into a normal rollover IRA account w/o those extra fees.
However, remember that if you are truly taking all legitimate deductions for your rental properties (including depreciation), you probably aren't showing a huge taxable income from it. If you are, then you must really know how to maximize those rents!
If you are posting for free advice on a web forum for retirement plans advice, I'm assuming you're not pulling in net income from your rentals of $150k/year...if you ever do get that high, though, and you're self-employed (with no non-immediately family employees), you can really kick things up a notch with your own Defined Benefit Plan (pension plan). The benefit isn't as great for a 30-year old, but if you're 45 or older, you can set yourself up a DBP to pay out a certain $ amount at a certain age, and then put in the necessary funds every year to accumulate the cash to be able to fund that pension payout. We're talking contributions of $100k/year up to well over $200k/year to the account for someone in their late 40s/50s/60s, depending on income.
The 2 "personal pension plan" drawbacks:
1) You will most certainly almost need a pension consultant to set up the plan and administer the plan each year. Maybe $4k to set it up, $2,500/year to maintain.
2) You have to file forms with the Pension Benefit Guarantee Corp. Oh, and once you set up the plan, you have to fund it each year, regardless of what your net income is. Which would suck if 5 of your tenants stopped paying rent, and you have a hefty loss for the year. You can delay pension plan contributions (just like some business do), but if your unfunded balance drops too much, you'll pay fines. It is possible to change the equation and variables after they're set up, but I imagine it's a royal pain in the ass with paperwork, and not something you can do on a whim every year.