Compact Double Options:
I had a 50/34 compact and 10sp 12-27 cassette. Because the availability of 10sp cassettes with very low gearing is so poor, I ended up using a Harris Century Special 9sp cassette (13-30; this is a modified Ultegra 9sp cassette). The 30t cog works with my Ultegra RD. And the short cage wraps just enough chain to make this workable with a 13-30 cassette and 50-34 crank. I worry a bit that if I hammer in the big-big combo that I'll break the RD, but I shouldn't be in this cross-chained gear anyway.

So the 34-30 combo gives a 29.8 gear inch granny. To use my 10sp brifter with the 9sp cassette, I'm using the Jtek Shiftmate. It helps, but the shifting isn't quite as crisp as same-speed Ultegra components would be. If I wanted to go lower on gearing, I could use a mtb 9sp cassette with a 32t or 34t large cog - but I'd need to get a mtb derailleur as well.

Triple Options:
You could go to a triple, but here too, the lack of low geared 10sp cassettes is a problem. The lowest Shimano 10sp cassette is what you already have (12-27), so with a typical triple crank with 30t granny ring, you can get down to 29.2 gear inches. If you want to go much smaller, you'll need to either go with a 9sp cassette or swap out the granny ring for a smaller ring.