Always keep in mind that on special occasions, you target a focused customer/client/employee base. While I won't recommend you sending a Valentine's Day gift to your client's girlfriend, a gift to your prospect's mom on Mother's Day will certainly make the prospect look at you when he needs similar service in future. Kind of indirectly asking your prospects to reach you, I have used this technique and it works. Though the prospect may know your intention, she/he will still like your approach of making their near and dears happy on a special occasion.
Coming to more special occasions, the festive season from Halloween to New Year, check out which holiday benefits your business most. If you are not sure, I would recommend New Year's Day or Christmas. The following part discusses this.
Buy items that are stable and can be retained for long. If you send in a box of chocolates, your client will empty it and throw the box into the dustbin. Choose something durable. The most common items are key-chains and pens. There are many websites that take in bulk orders and do not even charge for printing your company name and logo on it.
However, smaller items do not show off your company name clearly. They serve best as a keepsake or incentive for your staff. I would prefer something that shows my company logo from at least ten feet. Some examples are executive file folders, umbrellas, and backpacks or briefcases (depending on your budget: you can always negotiate with online stores).
Choosing such items help you minimum two-fold: the person receiving the gift will appreciate it and keep it with him/her. When the person goes out, she/he carries the gift along with. Imagine your company logo with contact information, traveling to the other end of the city as the person travels.
Another benefit I see from durable and appealing products is that they are not thrown out once they degrade. If your client/prospect/employee feels that the product has lost its luster, she/he will give it away to her/his family – continuing your once-invested advertising campaign, for years to come.
Consider just three points to begin: occasions and holidays, type of gifts, and your budget. You can always get an online gift shop that will give you discounts to help you promote your business in the holiday season.