I use 1 liter water or soda bottles filled with water for weights. Works great and chances are you already have some laying around. You can regulate the amount of weight you apply by the amount of water in the bottle.

Got to second the razor plane for shaping balsa blocks. Best basic inexpensive tool I have.

A good hinge slotting tool is also a must have. Makes short work of a tedious job. I use a Dubro set that has three different sizes and a small pick. I used to hate cutting hinge slots, now it's a piece of cake.

I also like the sticky-back sandpaper from Great Planes. Not only does it work well with their sanding blocks, it works well if you need to make any odd-ball sanding blocks. For instance; a small block for sanding between horizontal and vertical stabilizers, or adhered around a dowel for sanding concave surfaces.

Of coarse the must have if you plan on building a lot or for use when doing repairs is a Dremel moto tool or equivalent. On my bench, this tool gets used for a variety of tasks more than any other. They are one of the more expensive single tools you'll buy, but it's handy for things other than building.