Recommendations for building unit cohesion in a reserve unit?

Building unit cohesion in the Reserves definitely has additional challenges. I would recommend three things:

1) Make sure to take full advantage of your AT period, whenever that comes. You can build a lot more cohesion by going through the tasks and challenges together in a period that is longer than a single weekend.

2) Bring this up to your squad leader, and ask to speak with the platoon sergeant if you think he will be responsive. It's likely that they don't know that this perception is happening, or they may not know that it is affecting other people as much as it is.

3) Volunteer for individual and especially group work details where you will be near the other members of the detail. Often the best time to get to know others is while working together on a task, instead of just trying to force it in between work. Ask the other Soldiers about themselves... let them do most of the talking and learn about their background. They should naturally open up to you.

Challenge your fellow Soldiers to be the best; the place that people want to serve and non-hackers are brought to standard.
Build a unit identity - a motto, unit t-shirts, something you are known for. Have a unit coin. A watering hole that your Soldiers go to celebrate a promotion, welcome new members, or salute a departing one.

Try to initiate some sort of unit function and take the lead on it. It can be as simple as lunch time bowling or a contest of who can make the best Dallas Cowboys joke.
Try to find something challenging. I've seen cohesion grow most when you guys go through things together.
Volunteering is another function I do a lot. Find your local Habitat for Humanity and get on the calendar to go help build a house. It's a lot of fun and you'll learn a thing or two.

I'd echo what LTC Yinon Weiss said about PT. I had my folks do one day of "typical army pt" and one day for team building such as ultimate frisbee or something. Other things that bring a unit together are training at bases you travel to for BA when you all stay in a single building etc. I used to bring a satellite dish, speakers and a projector/screen and have movie nights, sports games, cards or whatever as well as unit bbqs and stuff. All are great for unit cohesion.

It all begins with the unit sponsorship program. When a new Soldier/officer joins the unit, make them feel welcome by meeting them at the front door. Show them around the unit and help them with inprocessing. Introduce them to the command team and tell them about your unit's history. Eat with them during chow and discuss any issues or concerns, and be certain to initially counsel them in writing about the unit's expecatiations and their expectation of the unit. Give them an additional duty task if they've got some time in to make them feel that they have some skin in the game and a part of the unit.

Reserves is an interesting mix compared to active duty. Active duty you see fellow soldiers for a 3-5 years stretch. In reserves most soldiers don't move geographically so you stay in units longer. There is a core group that I have served with in my unit for over 13 years, and some of those have been with the unit much longer than me.
Unit cohesion is hard but can be achieved. You have to work together, but also have fun together. We have two scheduled family events every year, family day picnic and a unit Christmas party. We also throw in a dining in/out over few years.
PT is a place to spice things up, we do monthly unit PT. We try to mix it up and not do the standard Army PT all the time. We have instructors in Martial Arts come in, Cardio Kickboxing, Combatives, commander runs, etc. On FTX the football comes out.
We mobilize all over the place and rarely together. Every few years we give deployment training to different sites that we were assigned. March - Brooke Army, April - Walter Reed, etc.
The officers all go to lunch together during drills which gives us a chance to bond and catch up.
Hope these ideas help.

Team-building activities where they have to rely on each other to accomplish common goals...I've used this method in the past and it worked pretty good.

Great thinking, easiest solution is - Deploy. You get together quickly when you are in the same boat and are hoping for the FRG to help your family!