The second option requires a bit more time and effort than a 301 redirect. You could do a mini overhaul of the site and turn it into a microsite for your main domain. This option is good for exact-match domains for your targeted keyword, and there are other reasons for going the microsite route that Rand's highlighted in his post about root domains, subdomains, subfolders and microsites. This strategy also works better if the old domain has decent rankings for the keywords you're targeting.
This is similar to house flipping, where a home is purchased and fixed up in order to sell for a fast profit. Unlike house flipping, however, there is essentially nothing to be done with the domain name in order to increase its value. Therefore, the key to a successful domain flip is being in the right place at the right time in order to acquire a valuable domain name before it is given a higher price.
You can add it as part of a private blog network or you can 301 redirect an expired domain to your site to bring some trust and authority to your site. Now, in my experience, the best place to look for expired domains is this free website called expireddomains.net. So, you just have to head over there, make a free account and this is the page that you’ll see when you log in.
If domains are real estate, then the same rules of real estate should apply. In real estate, taxes are based on the value of the property. So, a person asking $10,000 for a domain shouldn't be paying the standard $15-30 renewal fee as a person asking $200. The renewal fee should be based on the domain's value. Domainers want to claim it's like real estate without applying the most fundamental rules of real estate.
Using the keyword search feature you’d also be able to familiarize yourself with how much domains typically cost on average in a certain industry or niche. Even better, they have an addictive little app called “The Domain Game” available for iOS and Android. They basically give you a domain and you try to guess whether it sold for three, four, five or six figures. You get points for each correct answer and lose points for incorrect ones. You do this for a while and you start to see patterns. Your brain starts connecting the dots and you get better it. Now you’re ready to go out there and scout some domains!