Set of artist-grade watercolor/ watersoluble pencils; a small set is fine, because there will be demonstrator sets to try at class. Given the decision of whether to buy a smaller, high-quality set, choose that, rather than a larger-but-cheap set.The colors will be richer and more intense, and you will get much more pigment (and quality) per dollar.
Some suggested brands are (in alphabetical order): Caran d’Ache Museum Aquarelle watercolor pencils, Cretacolor Aquarelith (discontinued in 2008), Cretacolor Aquamonolith (woodless watercolor pencils), Cretacolor Marino, Derwent Signature Watercolour Pencils (discontinued in 2006), Derwent Inktense (watersoluble ink) pencils, Derwent Graphitint (pigmented graphite) pencils, Faber-Castell Albrecht Dürer, Lyra Aquarell, Sanford Prismacolor Watercolor Pencils, Van Gogh Watercolor Pencils (also discontinued). Note: If you have watersoluble wax pastels (which look like crayons), such as Caran d'Ache Neocolor II or NeoArt sticks, Derwent Inktense Blocks, Lyra Aquacolors, or Stabilotones by Stabilo (discontinued, but still “out there”), feel welcome to bring them. (Derwent released ArtBars watersoluble wax pastels in 2012 but discontinued them several years later.)
**Reference photos or items as a basis for art compositions. It is also helpful to have 3 or 4 prepared basic line drawings which can be developed into paintings.
Watercolor paper or quarter-sheets of regular-size watercolor paper, preferably at least 140 lb. Hot press paper is the smoothest and finest-textured, rough paper has the most "tooth", and cold-press and soft press (a Fabriano paper designation) are somewhere in between textures. (If you're not sure, bring it anyway!) The heavier-weight the paper is, the less chance there is of buckling. There will be some paper samples at the workshop.
Sketch pencil, such as #2 or HB drawing pencil. Also, a small strip of reusable adhesive (“poster putty”) is helpful for lifting colored transfer paper lines on your art surface, if you use transfer paper.
Eraser(s): a white vinyl eraser (such as the CLIC cartridge eraser or a battery-operated eraser) works well, as does a grey kneaded eraser. Avoid pink erasers, since they tend to stain the paper.
Whatever brushes you have; if you only invest in one brush, try a #6 round brush. White sable brushes are excellent and are actually synthetic. If you invest in a second brush, try a script/liner brush. (Cheap Joe’s Lizard Lick #2, also known as an Escoda brand reservoir liner brush, is wonderful.) Reservoir liner brushes are wispy, hold lots of fluid, and great for details. A 1" or larger wide, flat brush is great for large areas such as skies, should you want to try landscapes.
Optional: For someone without a big brush collection, I'd suggest a #6 round as a good, all-purpose brush. Another one would be an angle-shader brush (3/8", 1/2", or 5/8"), which is flat and angled and gets into tight spaces beautifully.
If you have more brushes, bring them!
Tissues, paper towels, and even Q-tips are nice for blotting.
Scrap paper for trying out color swatches. Also, sandpaper is useful for grating.
Two water containers, even if they are 2-liter bottles (or smaller) which have been cut down for this purpose.
Old toothbrush(es).
OPTIONAL: Masking fluid, plus some old (disposable, non-treasured) brushes for applying it.
OPTIONAL: Wax resist stick, like the clear waxy crayon found in Easter egg dye kits.
OPTIONAL: Plain, original-style (not the kind with Febreze) Mr. Clean Magic Eraser, which is effective for lifting watercolor pigment from Aquabord.