My mom likes to have solstice party every year. She doesn't do much decorating but has lots of candles of different colours around the house and fire outside near the lake. You could invite your neighbours over for solstice cocktails.

I think there are a lot of good suggestions in the thread for tasteful solstice decorations. I think a large lit up cross in a house is less an outburst of delightful Christian celebration than a lack of taste. What may be fun is to try and create tasteless solstice decorations. How about a tableau of pink flamingoes wearing holly and mistletoe carrying a banner saying "Welcome back Sun! Ra Ra Ra!"?