SEO is not as easy as it used to be. The market is very saturated now, content is now incredibly easy to generate and publish. Everyone and his dog (or gran!) can make a wordpress website. The main ranking factor for Google is still backlinks and will be for the foreseeable future. Your content might be better than your competitors but if they have more links they'll simply always outrank you and receive more traffic. Fair or not, its a fact. This method can see you get the strong backlink profile you need to rank #1. Please note a PBN is considered black hat so please diversify your backlink profile and do your research first.
Your course of action really depends on how much work and effort you can put into the expired domain. If you're barely able to maintain and optimize your current site, you probably want to just 301 redirect the old site (note: see my amended comment above about the link value not likely to be passed). If, however, you're more creative and have some time on your hands, you can try your hand at crafting a microsite. If you really know your stuff and are experienced at making money off various websites, you'd probably do well with the third option.
I'm running the new Mac edition of Domain Ronin and I have to tell you it's extremely stable. I'm getting about 250k pages every 5 minutes which equates to about 2700 checked domains. At the moment it's only using about 1 gig of memory for 100 threads. I've got the page count and other settings to the max. The builtin features like TF and spam checking on the fly are wicked helpful.
The easiest and least time consuming option is to 301 redirect the old domain to your existing site. This tactic obviously works best if both sites are in the same sector and are targeting the same keywords; otherwise, if you have a pet supply site and you buy an old Texas Hold 'Em poker site, a redirect probably might raise some eyebrows among the search engines. If, however, your site is and you buy and 301 redirect the domain over, you're inheriting a lot of topical and appropriate links.
This is a little bit on the grey hat/black hat side of the spectrum. But the fact is building a blog network with expired domains flat out works. The only issue is getting those cream of the crop domains without going broke. This video will walk you through tested strategies and techniques you can use to find powerful, yet affordable, expired domains.

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. 

The dropdate for the following TLDs is not verified. Domains might drop earlier or later: .eu .cat .travel .ae .af .as .at .aw .be .bg .bi .bj .bo .bw .cl .co .cx .cz .dk .dm .es .fi .fo .fr .gd .gg .gi .hk .hr .hu .ie .io .iq .ir .je .jp .ki .kz .lt .lu .lv .md .mg .mx .nl .nc .nf .no .nz .pe .pf .pl .pt .ru .sg .si .sm .sn .so .st .su .sx .to .uk .uz .za .berlin .cloud