Look, I gotta warn you - from your description there is no perfect solution, and I would personally suggest you stick with regular platform pedals. If you want more efficiency, but some BMX shoes or skate shoes (they have a non-compressible sole that's good for avoiding power loss from shoe cushioning while pedalling).

Regular Platform Pedals - advantages are that you can wear whatever shoes you want, including shoes that are comfortable to walk in. Disadvantage is that they're slightly less efficient - partially because padding on your shoe tends to be eat a tiny bit of the power, partially because you can't pull up while pedalling.

"Strap" Pedals - advantage is that you can pull up more, but still not quite as efficiently as with clipless. Drawbacks are that it's more difficult to get out of in an emergency stop (in my opinion) because you have to make the less-natural movement of pulling your foot back (whereas with clipless the clip-out movement is the more natural push-your-foot-to-the-side movement you usually do anyways). It can also get tedious to look down every time you put your foot in and out of the straps every time you stop (like at traffic lights).

"Clipless" pedals (two sided) - best power transfer - if you learn how to use them right. But it also isn't really a big increase vs regular pedals for most people. If you don't retrain your legs on how to pedal it's not even an advantage at all. They're also more expensive - $100 for pedals and $100 for shoes usually. Obviously the biggest drawback is needing to wear around specific bike shoes with a hard sole which isn't great to walk around in (I go grocery shopping in mine, but wouldn't want to walk around campus all day).

I've repeatedly found that you also have to spend time customizing where the clip is and how the shoe is setup to avoid knee pain. You might get lucky and be better, but it's easier with platforms.

"Clipless" pedals with a more walkable bike shoe - I tried this and hated it. You still have the clip on the bottom making noise on some surface when you walk, and the softer sole gave me knee problems while pedalling. Imo, this is the worst of both worlds.

"Clipless" pedals (one sided, with a platform on the other side) - I hated this to. You still have bike-specific shoes, and now I had to look down at the pedal every time I wanted to clip back in (some people say they don't need to, but I always found it to be a problem). The problem is that this is the worst time to be looking down at your pedals - it's always when you're going through an intersection where your head should be up looking at cars.

You can carry a 2nd set of shoes to walk around in and switch shoes when you get there this way (don't need to switch back for a short ride to a friends or the grocery store), but I just don't think it's worth it.

"Clip-on Platforms" - they sell something that sounds good - it's a platform that clips onto the pedal. Best of both worlds, right? Problem is, the platform clipping on is never as stable - it will twist around a bit (because the clipless part is mean to do that when you're wearing a shoe). I own some - don't waste your money on it.

"Clipless" (2 sided) with large platform that lets you use regular shoes - if you carry shoes to change into when you get there, this isn't the worst solution. In my experience, it's definitely better than the previous one. But it still doesn't solve your problem of need to carry 2 pairs of shoes.

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Bottom line - I ride clipless to work. But that's because I can keep a pair of shoes in my desk drawer and change into them when I get here. And I don't have any stops longer than grocery shopping on my way home.

I suggest for your purposes, best solution is to get two things -

1. A good platform pedal. Here's one my dad has that I like $30 (WELLGO Alloy BMX Mountain Bike Pedals Sealed 9/16") -
Amazon.com: Wellgo Alloy BMX Sealed Mountain Bike Pedal, 9/16-Inch, Black: Sports & Outdoors

It's sealed so it can handle being left outside in ****ty weather, the pins on it give you as much grip as humanly possible between your foot and the pedal (without going clipless or straps), and the wide platform should be good on your feet.

2. A pair of BMX shoes (or anything with a non-compressible sole, like Converse All Stars, or Skate Shoes). Here's one of the first ones that popped up on amazon -
http://www.amazon.com/DC-Mens-Black-...ords=bmx+shoes

BMX shoes are the easiest, as they're specifically designed for wearing on a bike with platform pedals.

I've gone through almost every iteration of possible shoe/pedal combinations, and I think it's just not worth the hassle for your needs to go with clipless. The speed/efficiency difference is very small, and likely to be eaten up by time spent messing with it anyways.