I'm a vegetarian, so I feel like sometimes people in my more traditional Jewish community are skeptical of coming to my house for Shabbat and holiday meals when they would usually be eating meat. With the tradition to have dairy meals on Shavuot, I feel like it's my time to shine. I really want to wow my guests, but I'm having trouble thinking of new dishes to try. Any suggestions?
The tradition to eat dairy on Shavuot has many possible origins and interpretations. One is that because the holiday celebrates the Israelites receiving the Torah, they didn't know the laws of keeping kosher before they received it and were thus unable to follow the rules associated with eating meat. Whether you ascribe to that sort of chronological gymnastics or not, the upshot is that people typically eat a lot of cheesecake on Shavuot, and it's hard to argue with that as a tradition. As you say, your guests will likely be expecting a dairy-heavy meal, and you don't want to disappoint.
While the Shavuot meal itself is often an afterthought to the cheesecake or ice cream sundaes for dessert, there's no reason to confine yourself to lasagna or macaroni and cheese. However, if you have a great lasagna or macaroni and cheese recipe and want a chance to show it off, there's also no reason to invent something more fussy/fancy just to impress your guests. (I happen to love both lasagna and mac and cheese, and I've served both at many a Shabbat and holiday meal.)
One way to go is to think about how you can add butter to your usual recipes or vegetables. Even the healthiest veggies are often improved by some butter. Though I've never actually done this myself, I've heard about some fantastic ideas for making your own compound butter (ignore the parts of this link that are obviously non-kosher). Artichokes with butter sauce is a good (and fancy!) bet, as is roasted acorn squash with a pat of butter and brown sugar inside.
Butter aside, eggplant parmesan appeals to a broad range of palates. If you're serving lunch, fresh mozzarella, tomato and basil on crusty bread is also a crowd-pleaser. Because it's a holiday and not Shabbat, you can cook, which opens up options like make-your-own pizza and made-to-order omelettes.
I have recently become kind of obsessed with spaghetti squash (leftover from using it as a substitute for pasta on Passover), and I just made something amazing: half a roasted spaghetti squash mixed with an egg, a cup of ricotta cheese, a handful of shredded mozzarella, a clove of grated garlic, salt and pepper, some defrosted and squeezed-out frozen spinach and, the magic ingredient, a pinch of nutmeg. Bake until it's brown on top. Oh. Yum.
Even with your focus on the main course, don't neglect dessert. Cheesecake and ice cream are obvious Shavuot favorites. Even if you go with store-bought, your guests will still be happy. These things are delicious, and a few pints of new Ben and Jerry's flavors will always be a conversation starter. If you're inclined to make your own ice cream, it can be a lot of fun and a great opportunity to experiment with flavor combinations.
Whatever you serve, you may want to consider leaving some Lactaid pills out just in case your guests weren't prepared for quite so much dairy.
Chag sameach, and be well,