I'm trying to think of a place to go for our 8 year anniversary. In the past, we've been to Hawaii, Bora Bora, NYC, SF, Grand Cayman and all over the Caribbean. We'd be going in May, but could always push it up to April to beat the summer crowds. We have a week and about $3k for plane tickets and hotel (not including food/spending money). We're definitely not lay on the beach all day people. Love food, exploring, outdoor activities. Thanks in advance!
Someone posted the other day about spring airfare to Spain at around 800pp. I'd pick one city or region (based on your interests) and rent an apartment or find a nice quiet inn. You could do airfare and lodging within that budget. Then plan little day trips (again, based on your interests) plus plenty of time to explore the city, eat good food, etc.
We went to Spain in early-mid May last year for our 5 year anniversary. It wasn't too crowded and the weather was perfect. You could rent a car and an apartment/house in the Andalucia region and spend a week exploring.