Do you have enough of your comic done to print? My issues/books have always been my top seller. People like content. See if you can get like a run of like 25 to test it out, maybe at most 50... whatever you feel comfortable with.
So, the thing with prints is, there are so many out there at this point that you kind of have to wow the customer (especially for big ones that take up a lot of space). I would suggest starting out smaller with mini or mid-size prints. Here are common sizes:
small - 4x6" or 5x7" (sell for $5)
medium - 8x10" or 8.5x11" (sell for $10)
large - 11x17", 12x18", 13x19" (sell for $15)
I recommend only doing one from each category, and like I said, maybe skip out on the big ones for now. Find a print shop in town (not office max or fedex, a legit one) and ask for quotes. You can also look at prices at Catprint.com, but you'd have to know what kind of paper weight and finish you want... I think it's better to feel it yourself, personally. Don't order blind. They might have a sample pack or you can just feel examples at the local print shop. (as a point of reference, I do 80lb satin, but there are both thicker and thinner varieties)
And only get like 5 of each piece to start out, especially if you're doing original only. You don't want to invest in a lot of inventory and then find out it doesn't sell. Treat it like a test market. This goes for any other merch you get, too, if you can help it. 10 of each thing at MOST.
Like others suggested, stickers are good starter items. Pinback buttons are good. Maybe find someone on Etsy that will do them for you since they'll have lower minimum quantities than big companies. These do require you to be able to use template files and work with transparency though, so make sure you have programs that support that.
I personally feel like acrylic charms are more intermediate, so I'd suggest trying out a few events first before investing into those... I do think your art would do well with them eventually, though!
I would shoot to have at least 5 designs of each item you're stocking. More if possible, but don't feel too much pressure your first time around. If you end up doing it more often, your inventory will grow naturally as you build on past years and figure out what works and what doesn't work.
One more important question, if you're going to print ANYTHING: do you know how resolution, DPI, trim lines, and bleed lines work? Because thaaat's important. Anything you make for merchandise has to be the correct resolution so it doesn't print blurry.
Another thing you can sell is commissions. These are a great way to pick up extra money if selling merch isn't doing it... just don't overload yourself. People will usually order it from you and expect to pick it up by the end of the day, so keep it simple. When I did them, I only did busts and had maybe 5 slots a day... but if you can do them quickly, you can have more slots or do more kinds of things. Just be careful not to have your head down drawing for too long, it makes you look unapproachable to customers Get their contact info, references, and money upfront.
Man there's so much to be said for vending, but I think I've been wordy enough for now. If you have any particular questions or want to learn more about something, feel free to ask!