Smoking weed with just one other person is good, I enjoy it.
I smoked a few times with a girl i liked. If you want to melt her heart, or get into her pants, take her to a place where you can watch the sunset while smoking a spliff. I took that girl to Manly Beach (i think, i forget... Short term memory is shot right now) to watch the sunset. It shows you have a romantic side, and the buds add a fun tone to it. Fish and Chips works well too. Most people don't care about people smoking weed.
As for people coming up to you etc, I've had this happen to me quite a few times while smoking in public. I was smoking in a park out of a 30cm glass Billy one time when this 20 year old guy comes up to me and goes to me in a stiff tone "Are you smoking marijuana?" and I just replied "Yeah we are mate, would you like us to move? I'm happy to if you want" and it caught him off guard, he nodded and walked off and that was that.
If their violent, which I've encountered in the past, just had over your bud. I never keep more than $50 in my wallet if I'm going out at night and if i do have to take more cash it goes in my shoe. I can't fight for shit on weed, which you'd expect. Which is kind of a shame because it means 8 years of martial arts is wasted when i go out and smoke, which is quite often.
You could carry a knife, but i wouldn't think of it as a good idea considering you could get caught by the pigs or you could scare the fuck out of the girl. If your having a picnic, just take a knife along with you, that is a perfect excuse for having a knife that you used to defend yourself, and you won't get in trouble for carrying it by the pigs if its in a basket, not in your pocket.
If your super desperate, you could always just put a line of honey oil on some ciggies, that way it won't smell at all.