I thought I'd take a few minutes to share a few tools in my sketch bag. When I'm not sketching digitally on my iPad, Here's a few tools I like to keep handy for sketching while I'm out and about. I've provided amazon affiliate links below for easy purchasing as well if you're in the market or want to try some new supplies.
A great little fountain pen that is a no mess solution and has a great feel in drawing is the Lamy Safari pen (Extra Fine Nib). It's a great option if you're looking to get something affordable yet a step up. Be sure to get the Lamy refill converter as well, so you can use the ink you desire (waterproof and your color of choice) or you'll be limited to the available non-waterproof cartridges.
Speaking of ink, I have found the Pilot Bamboo Charcoal Black a nice smooth black ink option. It's rich yet over done. You might want to experiment with a few brands of inks, but if you intend to use with watercolor paints, you will want waterproof ink.
The TWSBI fountain pen was my first. I was attracted to it because of the ease of use yet versatility. It does require more maintenance than the Lamy, but also has a well (that you can see how much ink is left if you get the clear barrel model) as opposed to a cartridge. It's a bit tricker to use but another great option if a full on fountain pen is intimidating.
There are certain moments that you will want to blend ink, so using a water-soluble ink will be your ticket. You can fill one of the pens above with such an ink, or if you'd like a different option check out the Tombow. It had two sides and is a BRUSH pen as opposed the extra fine nibs on the fountain pens. One side is like a brush, the other is like a fine tip marker.
I live for color, so I always have my Windsor and Newton Watercolor travel kit handy. There are a ew different sizes, depending on how many pans of color you want available. I suggest starting with the one I've listed as it's a little more budget friendly as well as more compact for travel. When the pans of color run down, they are easily replaced. You can even swap out colors if the stock colors aren't your colors of choice. This set is a staple of my travel bag. I never go anywhere without it.
And if you intend to use watercolors, you'll need a handy option for brushes that travel well. You don't want to be fiddling with brushes and water cans while on the go, so these water brushes are a necessity. This set has 4 size options which is also nice. You just unscrew the cap and fill with tap water at home, or refill on the go. Simple.
And Lastly, there's a myriad of options for your sketchbook, and a lot of it is personal preference. I do suggest however that if you are planning to use watercolors or wet media, get a sketchbook with watercolor paper. I've used the Moleskine Watercolor sketchbook. It's taken some getting used to the size, but it captures my watercolor nicely. I've also used other Moleskine (as well as other brands) that weren't watercolor paper but with varying degrees of success. It might come down to how wet the paint is.
The point is to get out there and start sketching! Try different supplies as your budget allows. Go with friends, and swap supplies. Now get out there and sketch!