If it passes code I'd put a 6 foot iron fence in the front yard with a lockeable gate. It's very common in Chicago and the near suburbs and it stops thieves cold because it's harder to scale since they have iron spikes at the top.

The one downside is if it's locked and your house is on fire it takes the fireman longer to get you out because they have to crack the lock. I live in a near suburb and we can no longer install new fences in the front for that reason but half the houses do have them and it definately as far as thievery goes is worth the investment.

My whole front yard is garden and I have few problems (surprisingly) with thievery.

Things I won't plant or put out front:

Tulips and Daffodils (I live near a school and they will get picked)

Fountains & Statuary - Neighbor across the street had their fountain stolen and is was big and they still snagged it.

Vegetables - These are for the back only. The last house I was in I had my prized egglants stolen from the front.

No pots - You're just asking for it. I have one and fully planted it weighs over 200 lbs. That's my one exception.

Another trick I use is: I always try to greet or start up a conversation with anyone I see looking at the garden. I try to make friends not enemies out of the school kids. Sometimes I will offer to cut some flowers for a child to take home to Mom. For the adults, I'll offer free starts of plants like my Irises or Coneflowers.

Usually, if people see you as a nice person rather an ogre then they're less likely to steal from you.

Hope this helps,

Spiny plants are going to be less attractive for obvious reasons, and if you put gravel around them it's going to be more audible (crunch, crunch) if someone's skulking around in your front garden in the small hours. Once I had a box bush pulled up but that was a kid's prank apparently. I wouldn't worry too much though, as long as you're not splashing out on some hugely expensive specimen plant.

Here is a link that might be useful: police advice for garden security