For each possible fund-raising exercise, consider its labour cost: a fund-raiser that consumes a lot of labour is detracting from other possible activities. Consider the up-front investment: buying and screen-printing a lot of T-shirts costs a lot of money. Consider the down-side: what if you spend hundreds of dollars on chocolate, and then no-one wants to buy it? (OK, bad example.)
The most common form of merchandising at the convention is T-shirts. The win-win here is that fans get a memento (plus each Swancon T-shirt, when worn appropriately, is worth a fannish credibility point), and the convention gets free advertising. Of course, only T-shirts sold before the convention are any use as advertising, so you'll want to have some on sale at the Swancon prior to yours.
Disadvantages of T-shirts are that they are not one-size-fits-all, involve a significant up-front investment, have a not-that-amazing markup, and you can get stuck with excess stock at the end of the convention. This can make them less attractive: each T-shirt you have left over could eat the profit from maybe three other shirts. It's tempting to knock out shirts cheap on the last day, but if you do that, you're hurting future conventions, by prepping fans to expect that if they wait to the last day, they can pick up a bargain. So it's best to order less shirts than you think you're going to sell. The more sizes, colours and styles you carry, the higher the chance that you will get stuck with unsaleable stock.
You may wish to take pre-orders for specific sizes, styles, etc. If you do this, be sure to specify a deadline by which ordered shirts must be picked up; after this time return them to general stock so you have some chance of flogging them.
There are some outside-the-box opportunities in T-shirt sales. For example, you could sell just the artwork, in an iron-on form. (Would work particularly well if the hotel has irons and ironing boards in the rooms.)
other kinds of merch are possible: bags, commemorative zines, etc.
Buy boxes of "clean-skin" wine, and knock 'em out to punters at a small mark-up. Though this does involve some cash outlay, it's usually for a short time, as you deliver the wine to punters within a few days of you buying it. Unless you make order delivery coincide with some other event (e.g. a quiz afternoon), there will be significant logistics involved in delivering the wine.
Nice because the markup is often 100%: buy each bar for say $1, sell 'em for $, however return for time invested can quite low unless selling large quantities from a small number of locations.
Discount coupon books
Needs people to acquire prizes, people to create questions and table quizzes, people to work the door, MC, mark, and collect money for raffles, heads'n'tails etc. All up quite an investment of labour, but we have to give away that L. Ron Hubbard dekalogy somehow.