A visit is enough, anything more seems staged.

Spring seeds?

Jacobs nailed this one!

I'm not a "pop-by" kinda guy.

Gee, I don't know, pop overs?

I did Brian Buffini around 15 years ago and have always loved the idea of pop bys.  However, I have not done them enough and your question has reminded that now is a good time to start again.

As a former Buffini-ite :-) this phrase that Brian coined has really brought rewards to me - it's a cute, fun, way to stay in touch with your previous clients as well as a great way to just say hello - there are all kinds of ideas on the internet - one that I've 'borrowed' for the winter is buying lip balms and putting 'don't let your friends or family get chapped in a real estate transaction' have them call me!

I don't do the lotto scratch-offs since they might not win - but I do get a little candy container  shaped like a four leaf clover, fille it with candy and add the note, "you'll always have the luck of the irish when you work with me."

You get the general gist - just go for it - doesn't need to be huge - sometimes dorky is better

 What you are doing has real texture and will strengthen the fabric that holds your community together.

Here is how to get a big bang for your effort. This suggestion will have enduring value if you can locate a NEW BUSINESS in the area, restaurant, pizza etc. I like yogurt shops.  TALK with the owner and suggest you will be happy to introduce the business's to your A list and farm. [BEAWARE, THE BUSINESS OWNER IS ACCUSTOMED TO PAYING FOR THIS SERVICE] What you need is the two for one coupons or prepay (at a 50% discount) for 25, or 50 or whatever you can afford or anticipate.  When you reach agreement...well, you gotta say, "I need to sample the goods!"

Now your Pop-By has purpose and value, and if you are smart a segue into the local real estate happenings. For instance in many of my farms the happening is the highest average sold prices in TEN years!   You say, "You have been very patient, now you can afford to move."

Find something with a St Patrick's Day theme--and bring it with a "tag" that says how lucky you are to have them as a client. I haunt the Dollar Stores--often you can find interesting little things for a dollar.

Will Hamm - flower seeds may be a good choice. Also 17th March is St Patrick's Day - so you may want 'Pot Of Gold' scratch lottery ticket? Just a thought.

Easter is in April - so it eggnog would be good for April.

I am with Larry in never heard "Pop-bys".  From the comments guessing unexpected 'drop in'   Magazines are well received if something they can use and read.  

I have never heard of Pop-bys, so I can't help you.

Depends on who your target is.  If it's women homeowners, then I would do plants or seeds or bamboo for spring.  Could also leave them w/ tips for spring cleaning.  Maybe team up w/  carpet cleaner to get a discount for your clients or the neighborhood.