A sweater that can approximate a blazer
An open-button sweater can look almost like a jacket, provided it's the appropriate type. We recommend a fine knit with a shawl collar, which can most closely approximate the look of a traditional suit jacket.
If you're feeling underdressed, add a tie that can hide underneath the front buttons of your sweater, just like it would on a jacket. Some manufacturers even sell "sweater blazers," which are exactly what they sound like: sweaters that are fashioned to look like blazers.
A sweater to wear underneath a blazer
If you'd like to do away with ties altogether, take a note from James Bond and get yourself a roll-neck sweater. There's just something inherently dressy about a turtleneck with a blazer over it, and if you have the neck to pull it off, there's no reason why you shouldn't give it a try.